Diane, since you don't give us the method from the textbook, I'll just show you.
All you do is multiply the percentage by "n", the number of ages.
.25(36) = 9. If we were to come up with a decimal, we would round up to the nearest whole number. Since we have a whole number, we count to the 9th age, which is 41.
Hope this helps!
Diane R.
12/31/15