Finding percentage increase,Given the change is the quantity/number over a span of time

The population of a town doubled every 5 years from 1960 to 1975. What is the percentage increase in population in this period?
a. 800                          b. 400              c. 700              d. 600

Solution :From 1960 to 1975, in 15 years population doubled every 5 yrs => three times
So, 2 X 2 X 2 = 8 times => 700% more.