first you need to write an equation or an inequality.
5+1.80x<=12
the x here is how many miles you are paying for, now you just solve for x.
5+1.80x<=12
-5 -5
1.80x<=7
/1.80 /1.80
x<=3.8
you can travel only three whole miles, you do not have enough money to go four full miles.