A tank fills at 7 liters per minute. How many minutes to fill 56 liters?

A tank fills at 7 liters per minute. How many minutes to fill 56 liters?

• A. 7
• B. 8
• C. 14
• D. 9

Answer: (B)

Think about rate as how much is added each minute. If 7 liters flow in per minute, then after t minutes you have 7t liters. You want 56 liters, so set 7t = 56 and solve for t. Dividing both sides by 7 gives t = 8. So it takes 8 minutes to reach 56 liters. You can check by adding 7 liters eight times: 7, 14, 21, 28, 35, 42, 49, 56. After 7 minutes you’d have 49 liters, and after 9 minutes you’d have 63 liters, so 8 minutes is the exact amount.

Think about rate as how much is added each minute. If 7 liters flow in per minute, then after t minutes you have 7t liters. You want 56 liters, so set 7t = 56 and solve for t. Dividing both sides by 7 gives t = 8. So it takes 8 minutes to reach 56 liters. You can check by adding 7 liters eight times: 7, 14, 21, 28, 35, 42, 49, 56. After 7 minutes you’d have 49 liters, and after 9 minutes you’d have 63 liters, so 8 minutes is the exact amount.