Practice Test $\backslash$#$17.$ Ricky runs $7$ miles each Saturday. If he doubles his usual speed, he can run the $7$ miles in one hour less than his usual time. What is his usual speed? A $2$ miles per hour B $2\mathrm{.}5$ miles per hour C $3$ miles per hour D $3\mathrm{.}5$ miles per hour E $7$ miles per hour