Advanced MathMediumDecay of 20% over two periods
Advanced Math practice question
A pond holds 500 litres. Each week 20% of the water evaporates and none is added. How many litres remain after 2 weeks?
Answer: 320
500 × 0.8² = 500 × 0.64 = 320.
Common wrong answers
- If you answered 20
- 20 is 500 × 0.2², keeping 20% instead of 80%.
- If you answered 300
- 300 subtracts 20% of 500 twice (linear).
- If you answered 400
- 400 is after ONE week.
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