If 5 pounds of apples cost $4, how much would 15 pounds of apples cost?

Set up a proportion: 5 pounds / $4 = 15 pounds / x. Cross-multiply: 5x = 60. Divide: x = $12. Alternative thinking: 15 pounds is 3 times 5 pounds, so the cost is also 3 times: 3 × $4 = $12. Either method works. Proportions are central to many ASVAB problems: cooking (recipe scaling), maps (distance scale), construction (mixing ratios), and any 'if X equals Y, then how much for Z' situation. Set up as fraction equal to fraction with corresponding units in the same position. Cross-multiply to solve: (a/b) = (c/d) means ad = bc. The unit-rate method also works: $4 ÷ 5 pounds = $0.80 per pound; $0.80 × 15 pounds = $12. Use whichever method clicks for the problem. Verify the answer is reasonable — 15 pounds should cost roughly three times 5 pounds.