$-$ Mutual Fund Fee Structure

The Investments Fund sells Class A shares with a front$=$end load of $6\%$ and Class C shares with $12$b$-1$ fees of $0\mathrm{.}5\%$ annually as well as back$-$end load fees that start at $5\%$ and fall by $1\%$ after each full year the investor holds the portfolio. Assume the portfolio rate of return net of operating expenses is $10\%$ annually and the expense ratio has already been factored into the $10\%$ rate.

a$)$ If you plan to sell fund after four years, are Class A or Class C shares the better choice for you?

b$)$ What if you plan to sell after $15$ years?

Please explain well and show computations!!