How many feet equal 3 miles?

In a mathematical approach, you can use a proportion. The easy way is to do what I'm doing now though, which is basically a proportion.

There are 5280 feet in 1 mile. There are "x" feet in 3 miles. To get from 1 mile to 3 miles, you multiply the number of miles by 3 (1 mi. * 3 = 3 mi.). You do the same thing with the feet (5280 ft. * 3 = 15840 ft.).

So there are 15840 ft. in 3 mi.

Hope this helps. :D