What pattern does this problem use?

You're building an expense-splitting app. Given a list of purchase amounts and a target reimbursement total, find two purchases whose amounts add up to exactly that total and return their positions in the list. You can assume exactly one valid pair exists and the same purchase can't be used twice. Example: amounts = [12, 35, 8, 47], target = 43 → [0, 3] (because 12 + 31... wait, 12 + 31 is wrong — 35 + 8 = 43, so [1, 2])