Andrea needs 45 rhinestones to finish an art project. She bought a third of what she needed and found a fifth of what she needed in her supplies. How many rhinestones does she still need? Think carefully first, then make a decision:
Andrea bought 45 / 3 = 15 rhinestones. She found 45 / 5 = 9 rhinestones in her supplies. Thus, Andrea still needs 45 - 15 - 9 = 21 rhinestones. So, the answer is 21.