The general Formula for a decreasing Annuity is:

For this example we have:
P = 78,000
n = 7*4 = 28
i = 8%/4 = 2% = 0.02
After substituting you can find value for R, rent.
Answer:
0.300 + 0.030 + 0.004
or
0.100 x 3 + 0.01 x 3 + 0.001 x 4
I think your answer would be 4,000
We already know that C = 45. All we really need to do is simplify the equation and then solve for x.
45 = x + 0.06x + 0.08x + 0.1x
45 = (1 + 0.06 + 0.08 + 0.1)x
45 = 1.24x
x = 36.2903, which rounds to $36.29 for the haircut.
Range is the highest value minus the lowest value