Question 818271
In 1 second he can run 5 times what he can run in 1/5th of a second, and in 20 seconds he can run 20 times what he can run in 1 second.
If he can run 10 feet in 1/5 of a second, 5 times that is 50 feet (that's what he can run in 1 second). And 20 times that is 1000, or what he can run in 20 seconds. 
To solve the problem you multiply 10 * 5 * 20 or 10 * 100 (because 5 * 20 is 100).
I don't think a dog run that fast though.