It would just be the tot foot time the cost
7.5 X 0.50 = $3.75

This result is actually true for any exterior angle. The exterior angle of a triangle is equal to the sum of the two remote angles, and above is a short proof of it.
7.2>0.9(n+8.6)
multiply 0.9 by all in parentheses
7.2> (0.9*n) + (0.9*8.6)
7.2> 0.9n + 7.74
subtract 7.74 from both sides
-0.54> 0.9n
divide both sides by 0.9
-0.6> n
ANSWER: -0.6> n
Hope this helps! :)