Question

the population of district a is three times the average population of the other nine districts in the city. what percent of the city's population lives in district a? any idea?

Answer 1

Let the average of the other nine cities be x, then the population of the nine districts is 9x. The total population of the city is a + 9x. But a = 3x. Hence, the total population is 3x + 9x = 12x.
Therefore, the percent of the city's population living in district a is 3x/12x * 100 = 25%