AlgebraMediumLinear model from a rate: when a quantity runs out
Algebra practice question
A pond holds 18 centimetres of water and loses 0.75 centimetres of depth per day in a dry spell. If no rain falls, after how many days will the pond be empty?
Answer: 24
Depth = 18 − 0.75d. Set 18 − 0.75d = 0: d = 18 ÷ 0.75 = 24.
Common wrong answers
- If you answered 12
- 12 is the depth after 8 days, not a number of days.
- If you answered 13.5
- 13.5 is 18 × 0.75, multiplying instead of dividing.
- If you answered 17.25
- 17.25 is the depth after one day.
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