Question. A charcoal grill burns fifteen coals to ash every twenty minutes of grilling. The grill ran for long enough to burn three bags of coals. Each bag of coal contains 60 coals. How long did the grill run?
Answer. 240
What step-by-step reasoning justifies that answer?
Reasoning: The grill burned 3 * 60 = 180 coals. It takes 20 minutes to burn 15 coals, so the grill ran for 180 / 15 * 20 = 240 minutes.