An ostrich can run 5 mph faster than a giraffe. an ostrich can run 5 mi in the same time that a giraffe can run 4 mi. find the speed of each animal.

The first thing we must do in this case is to define variables.
 We have then:
 x: speed of the giraffe
 x + 5: speed of the ostrich
 
 By definition we have:
 d = v · t
 d / v = t
 Where,
 d: distance
 v: speed
 t: time
 Since the time is the same then we have:

 5 / (x + 5) = 4 / x
 5x = 4 (x + 5)
 Clearing x we have:
 5x-4x = 20
 x = 20 mph (speed of the giraffe)
 x + 5 = 20 + 5 = 25 mph (speed of the ostrich)
 
 20 mph (speed of the giraffe)
 25 mph (speed of the ostrich)