Help tires rated for 60,000 miles and tread depth of 3/8" do I estimate how much tread is worn off for every mile driven?
Problems like this often confuse because of the fraction and the large number; I often make up a similar problem without all of the distraction. For example, if we had a tire that had a tread depth of 1" and it was good for 2 miles, then 1"/2 mi = 1/2 inch/mile. You can prove that by 1/2 inch/mile x 2 miles = 1" tread worn off.
So the obvious answer for this problem is 3/8"/60,000 mi = 3/8 x 1/60,000 = ? which is a very small number; i.e., something like a few micrometers (old name of micron)