AlgebraVery hardTime to reach a level with inflow and outflow
Algebra practice question
A tank holds 200 litres. Water drains out at 4 litres per minute while a pipe adds water at 1.5 litres per minute. After how many minutes will the tank hold 50 litres?
Answer: 60
The net change is −2.5 litres per minute: 200 − 2.5t = 50 gives t = 60.
Common wrong answers
- If you answered 80
- 80 is 200 ÷ 2.5, the time to empty the tank, not to reach 50 litres.
- If you answered 100
- 100 uses 1.5 litres per minute, the inflow alone.
- If you answered 37.5
- 37.5 uses the drain rate alone, ignoring the inflow.
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