how do i convert

15 miles = 15 miles x ( 5280 feet / mile   =  79 200 feet

1 hour  =  1 hour x ( 60 minutes / hour ) x ( 60 seconds / minute )  =  3600 seconds

15 miles per hour  =  79 200 feet per 3600 seconds  =  22 feet per second

"per" means "divided by"

Geno, you're right! My bad :(

Mike is riding his bike at 15 miles perhour.  What is his speed in feet per second?  (Use 5280 feet in each mile) 

You already have your mathematically computed answer; I'm just going to tell you a way to get an estimate. 

When you have miles per hour, multiply it by 1.5 to get feet per second.  In this problem the estimate is 15 x 1.5 = 22.5. 

The accurate answer geno gave you was 22 , this estimate method gives you 22.5.  Close enough for government work. 

_.

so he rides at 15*5280 feet per hour and there are 3600 seconds in an hour so we ahve the equation 15*5280/3600 feet per second=22 feet per second.