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Arithmetic progression
The population of a city increases by 700 each year. If the population grows to 25,000 in five years, what was its initial population?
Explanation and memory cue
The population increases by 700 each year for 5 years, so total increase = 700 × 5 = 3500. Given the population after 5 years is 25000, the initial population = 25000 - 3500 = 21500.