AlgebraHardPredicting from a linear trend
Algebra practice question
A reservoir held 20 million litres at the start of a rainy season and 50 million litres after 3 weeks, filling at a constant rate. If the rate continues, how much will it hold after 7 weeks?
- A. 90 million litresCorrect
- B. 70 million litres
- C. 100 million litres
- D. 110 million litres
Answer: A. 90 million litres
Rate: (50 − 20)/3 = 10 million litres per week. After 7 weeks: 20 + 70 = 90.
Why the other answers are wrong
- B. 70 million litres
- 70 is 10 × 7, forgetting the starting 20.
- C. 100 million litres
- 100 comes from 50 + 10 × 5, miscounting the remaining weeks as 5 instead of 4.
- D. 110 million litres
- 110 adds 20 to 90 twice.
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