Can someone help me work through this probability problem?? please and thank you

Question

Can someone walk me through how to solve this problem? I keep getting it incorrect and I don't understand what I'm doing wrong (please show work).

"The time (in minutes) until the next bus departs a major bus depot follows the distribution with f(x)= 1/20 where x goes from 25 to 45 minutes.

Find probability that the time is at most 40 minutes (must enter as fraction)"

Answer 1

1/20 * (40 - 25) = 1/20 * 15 = 15/20 = 3/4