We set up the cost function C(m) where m is the number of miles:

C(m) = Cost per mile after first mile * m + Cost of first mile

C(m) = 0.8(m - 1) + 1.2

C(m) = 0.8m - 0.8 + 1.2

C(m) = 0.8m - 0.4

We want to know m when C(m) = 20.75

0.8m - 0.4 = 20.75

Typing this equation into our math engine, we get:

m = 26.4375

The maximum distance we can ride in full miles is

**26 miles**