I'm trying to learn reduction. I have this problem called "even subset sum" that's very similar to subset sum. It's the same problem as as subset sum except that the only numbers allowed are even positive integers. I'm trying to show this problem is NP-hard by reducing the problem subset sum to it but I'm not sure where to start.
Hint: suppose that we are able to solve your problem efficiently. Then what if we take an arbitrary subset sum problem and multiply all elements by $2$, as well as the target?