If the population of a town decreases by 25 percent and then by 40 percent in two successive years, what shall be the total percentage decrease in the population after 2 years?

The total decrease over two years is 55%.

Successive decreases multiply the surviving fraction: (1 - 0.25)(1 - 0.40).

This equals 0.75 x 0.60 = 0.45 of the original.

The population left is 45%, so the decrease is 100% - 45%.

The net decrease is 55%.