AlgebraMediumTime for a linearly shrinking quantity to reach a value
Algebra practice question
A candle 25 centimetres tall burns at a constant rate. Two hours after it is lit, it is 21 centimetres tall. How many hours after it is lit will the candle be 10 centimetres tall?
Answer: 7.5 or 15/2
It loses 4 cm in 2 hours, so 2 cm per hour: height = 25 − 2t. Set 25 − 2t = 10: t = 7.5.
Common wrong answers
- If you answered 5
- 5 comes from 10 ÷ 2, the time to burn 10 cm rather than to reach 10 cm.
- If you answered 15
- 15 is the number of centimetres burned, not the time.
- If you answered 12.5
- 12.5 is when the candle would be gone (25 ÷ 2).
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