## Need help with a Trig question

Ok, Im kinda confused on a trig question I have for homework. Usually the answers are in the back to check the work, but not this one. Here is the question:

The diameters of the front and rear tires of a tractor are 36 in and 64 in respectively. Find the RPM of the front and rear tires when the tractor is traveling 9 miles/hour.

Now, here is my figuring:

9 miles 1 5280 12 1 rev 1 hour
---------- * ---- * -------- * ----- * ------- * ---------
1 hour 18in 1 mile 1 ft 2PI 60minutes

So that reads 9 MIles over 1 Hour * 1 over 18 inches * 12 inches over 1 foot * 1 Rev over 2PI * 1 hour over 60 minutes. The 1 Rev over 2PI tranfers the answer to angular velocity needed for the RPM measurement, the 1 hour over 60 minutes gets rid of the hour under the 9 Miles, the 5280 ft gets rid of the miles and transfers to feet, and then the 12 inces over 1 ft gets rid of the feet. NOW, the answers I got were
84 rpm for the small wheel 36 in diameter
47.2rpm for the large wheels 64in diameter.

Am I right or totally off base?