MathLibrary.sol

// SPDX-License-Identifier: MIT
pragma solidity 0.8.9;

library MathLibrary {
    function add(uint a, uint b) external pure returns(uint){
        return a + b;
    }
}

contract A {
    uint _a = 20;

    function getA() public view returns (uint){
        return _a;
    }

    function getSumA(uint a, uint b) public pure returns (uint sum){
        sum = MathLibrary.add(a, b);
    }
}

contract B {
    function getSumB(uint a, uint b) public pure returns (uint sum){
        sum = MathLibrary.add(a, b);
    }
}