Convert miles to feet by multiplying your speed in miles per hour by 5,280
Five tomatoes (sounds like 5,280) is how to remember that number in the future.
15* 5,280 = 79200 feet
Convert to feet per minute
Decide your speed in feet per hour by 60
79300/60 (1hour=60 minutes) = 1321.7
Convert to feet per second
Decide your speed in feet per minute by 60
1321.7/60=22
Your answer is 22 feet per second
Hopefully this helps