AlgebraHardWhen a descending lift reaches the ground
Algebra practice question
A lift descends at a constant rate. It is 120 metres above the ground 10 seconds after it starts and 60 metres above the ground 25 seconds after it starts. How many seconds after it starts does the lift reach the ground?
Answer: 40
The rate is (60 − 120)/(25 − 10) = −4 metres per second. From 60 m at t = 25, reaching 0 takes 15 more seconds: t = 40.
Common wrong answers
- If you answered 15
- 15 is the time from t = 25.
- If you answered 30
- 30 is the time from t = 10, not from the start.
- If you answered 35
- 35 uses a rate of −6 from 120 m at t = 10 by mistake.
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