ExternalCall.sol

pragma solidity ^0.4.24;

contract A {
    uint256 public n = 1;
    
    function setN(uint256 _n) public {
        n = _n;
    }
}

contract B {
    A public a;
    
    constructor(address _a) public {
        a = A(_a);
    }
    
    function setN(uint256 _n) public {
        a.setN(_n);
    }
}