AlgebraVery hardWhen a draining tank is empty
Algebra practice question
The depth of water in a tank decreases linearly. It is 40 centimetres at t = 2 minutes and 25 centimetres at t = 8 minutes. At what value of t, in minutes, will the depth be 0?
Answer: 18
The rate is (25 − 40)/(8 − 2) = −2.5 cm per minute. From 40 cm at t = 2, it takes 16 more minutes to reach 0: t = 18.
Common wrong answers
- If you answered 10
- 10 comes from using 40 cm at t = 0.
- If you answered 14
- 14 uses a rate of −2.5 from t = 8 with 25 cm, but 25/2.5 = 10, giving t = 18, not 14.
- If you answered 16
- 16 is the time after t = 2, not the value of t.
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