A trout population grew from 4 in 1992 to 16 in 1993 and then to 64 in 1994. If it continued to grow exponentially, approximately how many trout were there in 1995?

100

128

256

320

The correct answer is 256. From 1992 to 1993 the trout population increased to the square of the size of the population in 1992 and from 1993 to 1994 it increased to the cube of the size of the population in 1992. If it continued to grow exponentially, in 1995 it would increase to the the size of the population in 1992 to the fourth power, which is 256.