Call to Math with solidity "call"

Call.sol

pragma solidity ^0.4.24;

contract Math {
    // Answer
    uint public ans;
    // Module #0
    function plus(uint _n1, uint _n2) public { ans = _n1 + _n2; }
    // Module #1
    function minus(uint _n1, uint _n2) public { ans = _n1 - _n2; }
    // Module #2
    function multiply(uint _n1, uint _n2) public { ans = _n1 * _n2; }
    // Module #3
    function divide(uint _n1, uint _n2) public { ans = _n1 / _n2; }
}

contract Call {
    // Local Answer
    uint public ans;
    // Target module
    address public target;
    // Target module setter
    function setTarget(address _tar) public { target = _tar; }
    // Module caller
    function callPlus(uint256 x,uint256 y) public { 
        if (!target.call(bytes4(keccak256("plus(uint256,uint256)")),x,y)) revert();
    }
    function callMinus(uint256 x,uint256 y) public { 
        if (!target.call(bytes4(keccak256("minus(uint256,uint256)")),x,y)) revert();
    }
    function callMultiply(uint256 x,uint256 y) public { 
        if (!target.call(bytes4(keccak256("multiply(uint256,uint256)")),x,y)) revert();
    }
    function callDivide(uint256 x,uint256 y) public { 
        if (!target.call(bytes4(keccak256("divide(uint256,uint256)")),x,y)) revert();
    }
}