AlgebraVery hardInitial value from two later readings of a linear model
Algebra practice question
A tank drains at a constant rate. Two minutes after the drain opens it holds 180 litres, and seven minutes after it holds 105 litres. How many litres did the tank hold when the drain opened?
Answer: 210
It loses 75 litres in 5 minutes, so 15 litres per minute. Two minutes before the first reading it held 180 + 30 = 210.
Common wrong answers
- If you answered 165
- 165 goes the wrong way from 180.
- If you answered 195
- 195 adds only one minute's worth.
- If you answered 225
- 225 adds three minutes' worth instead of two.
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