Assuming that n = 0 when we focus on the first year, C = $48*1.5^n would represent the cost for the 2nd, 3rd, 4th, .... , years.
Check: $48(1.5)^0 = $48(1) = $48 (correct)
$48(1.5)^1 = $48(1.5) = $72 (correct)
$48(1.5)^2 = $48(2.25) = $108
These results are as expected.