Solve the Problem $n$ Payments Is Paid Off $k$ Payments Ahead of Schedule, the Amount of Unearned Interest

Solve the problem.
-When a loan scheduled to run $n$ payments is paid off $k$ payments ahead of schedule, the amount of unearned interest $\mathrm{u}$ is given by $\mathrm{u}=\mathrm{f} \cdot \frac{\mathrm{k}(\mathrm{k}+1) }{\mathrm{n}(\mathrm{n}+1) }$ , where $\mathrm{f}$ is the total scheduled finance charge. The finance charge on a loan taken out by Ximena is $\$ 668$ . If there were 24 equal monthly installments needed to repay the loan, and the loan is paid in full with 15 months remaining, find the amount of unearned interest. (Round your answer to the nearest cent.)

A) $\$ 87.13$
B) $\$ 267.20$
C) $\$ 100.20$
D) $\$ 254.13$