Solve/prove using gredy algorithm, show work please, thanks!

3. Suppose you are given two sets A and B, each containing n positive integers. You can choose to reorder each set however you like. After reordering, let ai be the i-th element of set A, and let bi be the i-th element of set B. You then receive a payoff of _lali. Give an algorithm that will minimize your cost. Prove that your algorithm minimize the cost and state its running time