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Destroyable.sol

import "./Ownable.sol";
pragma solidity 0.5.12;

contract Destroyable is Ownable{
    function close() public onlyOwner { //onlyOwner is custom modifier
    address payable ownerpayable = address(uint160(owner));
        selfdestruct(ownerpayable);  // `owner` is the owners address
    }    
}

Ownable.sol

pragma solidity 0.5.12;

contract Ownable{

    address public owner;

    modifier onlyOwner(){
        require(msg.sender == owner);
        _; //Continue execution
    }
    
    constructor() public{
        owner = msg.sender;
    }

}

People.sol

import "./Ownable.sol";
import "./Destroyable.sol";
pragma solidity 0.5.12;


contract HelloWorld is Ownable, Destroyable{

    struct Person {
      uint id;
      string name;
      uint age;
      uint height;
      bool senior;
    }

    event personCreated(string name, bool senior);
    event personDeleted(string name, bool senior, address deletedBy);
     //Create an event called PersonUpdated that is emitted when a persons information is updated. The event should contain both the old and the updated information of the person. 
    event PersonUpdated(string nameold, string name, uint ageold, uint age, uint heightold, uint height) ;

    uint public balance;

    modifier costs(uint cost) {
        require(msg.value >= cost);
        _; //Continue execution        
    }



    mapping (address => Person) private people;
    address[] private creators;

    function createPerson(string memory name, uint age, uint height) internal {
        require(age < 150, "Age needs to be below 150");
        balance += msg.value;
        //This creates a person
        Person memory newPerson;
        newPerson.name = name;
        newPerson.age = age;
        newPerson.height = height;

        if(age >= 65){
           newPerson.senior = true;
       }
       else{
           newPerson.senior = false;
       }

        insertPerson(newPerson);
        creators.push(msg.sender);

        assert(
            keccak256(
                abi.encodePacked(
                    people[msg.sender].name,
                    people[msg.sender].age,
                    people[msg.sender].height,
                    people[msg.sender].senior
                )
            )
            ==
            keccak256(
                abi.encodePacked(
                    newPerson.name,
                    newPerson.age,
                    newPerson.height,
                    newPerson.senior
                )
            )
        );
        emit personCreated(newPerson.name, newPerson.senior);
    }
    function insertPerson(Person memory newPerson) private {
        address creator = msg.sender;
        people[creator] = newPerson;
    }
    function getPerson() public view returns(string memory name, uint age, uint height, bool senior){
        address creator = msg.sender;
        return (people[creator].name, people[creator].age, people[creator].height, people[creator].senior);
    }
    function deletePerson(address creator) public onlyOwner {
      string memory name = people[creator].name;
      bool senior = people[creator].senior;

       delete people[creator];
       assert(people[creator].age == 0);
       emit personDeleted(name, senior, owner);
   }
   function getCreator(uint index) public view onlyOwner returns(address){
       return creators[index];
   }

   function withdrawAll() public onlyOwner returns(uint) {
       uint toTransfer = balance;
       balance = 0;
       msg.sender.transfer(toTransfer);
   }
  

}

Safemath.sol

// SPDX-License-Identifier: MIT

pragma solidity 0.5.12;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     *
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        uint256 c = a - b;

        return c;
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     *
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return div(a, b, "SafeMath: division by zero");
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        return mod(a, b, "SafeMath: modulo by zero");
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts with custom message when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

token.sol

import "./Ownable.sol";
import "./Safemath.sol";
pragma solidity 0.5.12;

contract ERC20 is Ownable{
    
    using SafeMath for uint256;    

    mapping (address => uint256) private _balances;

    uint256 private _totalSupply;

    string private _name;
    string private _symbol;
    uint8 private _decimals;

        constructor (string memory name, string memory symbol) public {
        _name = name;
        _symbol = symbol;
        _decimals = 18;
    }

    function name() public view returns (string memory) {
        return _name;
    }

    function symbol() public view returns (string memory) {
        return _symbol;
    }

    function decimals() public view returns (uint8) {
        return _decimals;
    }

    function totalSupply() public view returns (uint256) {
        return _totalSupply;
    }

    function balanceOf(address account) public view returns (uint256) {
        return _balances[account];
    }

    function mint(address account, uint256 amount) public onlyOwner{
        require(account != address(0), "mint to zero address");
        
        _totalSupply = _totalSupply.add(amount);
        _balances[account] = _balances[account].add(amount);
    }

    function transfer(address recipient, uint256 amount) public returns (bool) {
        require(recipient != address(0), "transfer to the 0 address");
        require(_balances[msg.sender] >= amount, "insufficient funds");
        
        uint toTransfer = _balances[msg.sender];
        _balances[msg.sender] = toTransfer.sub(amount);
        _balances[recipient] = _balances[recipient].add(amount);
        
        return true;
    }
}

underflow.sol

pragma solidity 0.5.12;

import "./Safemath.sol";

contract underflow{
    
    using SafeMath for uint256;
    
    uint256 n = 0;
    
    function add() public returns(uint256){
        n = n.add(1);
        return n;
    }
    function subract() public returns(uint256) {
        n = n.sub(1);
        return n;
    }
}

Workers.sol

import "./People.sol";

pragma solidity 0.5.12;


contract Workers is HelloWorld{
    mapping(address => uint) salary;
    
    function createWorker(string memory name, uint age, uint height) public {
        require(age < 75);
        createPerson(name, age, height);
        salary[msg.sender] = 25000;
    }
    function fireWorker() public {
        deletePerson(msg.sender);
    }
     
}