Use the AP formula to find number of hours.
Sum = N/2 * [2*F + D*(N - 1)]
4500 =N/2 * [2*675 + (-50*(N-1))], solve for N
Solve for N:
4500 = 1/2 N (1350 - 50 (N - 1))
4500 = 1/2 (1350 - 50 (N - 1)) N is equivalent to 1/2 (1350 - 50 (N - 1)) N = 4500:
1/2 N (1350 - 50 (N - 1)) = 4500
Multiply both sides by 2:
N (1350 - 50 (N - 1)) = 9000
Expand out terms of the left hand side:
1400 N - 50 N^2 = 9000
Divide both sides by -50:
N^2 - 28 N = -180
Add 196 to both sides:
N^2 - 28 N + 196 = 16
Write the left hand side as a square:
(N - 14)^2 = 16
Take the square root of both sides:
N - 14 = 4 or N - 14 = -4
Add 14 to both sides:
N = 18 or N - 14 = -4
Add 14 to both sides:
N = 10 hours or N = 18[discard]