test.sol

pragma solidity ^0.4.0;

contract MyToken { 
    
    mapping(address => uint256) public balances;
    uint256 public totalSupply_;
    
    constructor (address addr) public { 
        totalSupply_= 10000; 
        balances[addr] = totalSupply_; 
        
    } 
    
    function transfer(address _to, uint256 _value) public { 
        require(_value <= balances[msg.sender]);
        
    }
    
}

contract D { 
    
    function doTransfer(address newTokenHolder, MyToken erc20Toekn) external { 
        erc20Toekn.transfer(newTokenHolder, 20);	
        
    } 
    
}