Question 284297
let desi's speed be x mph

Fred's speed = x-0.5mph

In 5 hours desi has walked 5x miles

In 5 hours fred has walked 5(x-0.5) miles = 5x-2.5

5x+5x-2.5 = 20

10x=17.5

x=7.5 mph  Desi's speed

Fred's speed will be 7 mph