Arabbit can run short distances at a rate of 35miles per hour. a fox can run short distances at a rate of 21 miles per half hour. which animal is faster, and by how much?

Since the fox's rate is based on half an hour and the rabbit's rate is based on a whole hour, you would need to create an equation that is spanned on the same amount of time. 

To do this you would, simply, multiply the fox's rate by 2.

21 x 2 = 42
x 2 = 1

42 mi/1 h

A fox can run 7 miles per hour faster than a rabbit can. 

I hope this helps!