Discussion
The population of a city increases by 20% in the first year, it increases by 40% in the second year and due to Corona virus, it decreases by 10% in the third year. What will be its population after 3 years, if today it is 20,000?
Answer: B
The population after three years is 30240. Apply the changes in order: \[ 20000 \times 1.2 \times 1.4 \times 0.9 = 30240 \] The first year gives 24000, the second 33600, and the 10% fall in the third gives 30240.
24000 is the figure after year one only, and 33240 and 36940 do not come from these multipliers; 33600 would be the population before the third-year fall.
Because percentage changes multiply and do not add, a 20 per cent rise, a 40 per cent rise and a 10 per cent fall do not equal a net 50 per cent rise.
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