Variation of knapsack problem
This is called the subset sum problem.  There's lots written about the problem; go read about it.

How you could have figured this out on your own: you already know it is a variant of the knapsack problem, so if you had read the Wikipedia page on the knapsack problem, you would have encountered a description of the subset sum problem in the section of that page on variations of the knapsack problem.  In the future, I encourage you to do  research before posting, to make sure your question isn't already answered in the obvious places (e.g., Wikipedia, textbooks).