A tank holds 280 litres. A tap fills it at 12 litres per minute, but a drain at the bottom empties it at 5 litres per minute. Both are on and the tank starts empty. How long does it take to fill?
Answer and explanation
Correct answer: C. 40 minutes
- Step 1: The net rate is 12 - 5 = 7 litres per minute. Step 2. 280 ÷ 7 = 40 minutes. Answer: 40 minutes. Why not the others: 23 minutes 20 seconds: This ignores the drain and uses 280 ÷ 12. 56 minutes: This uses only the drain rate, 280 ÷ 5. 16 minutes 28 seconds: This adds the rates, 280 ÷ 17, as if the drain were also filling the tank.
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