Lissa, the owner of a health food store, was advanced $$15000$ by an investor. She signed a promissory note that stated the conditions of the loan: interest will accumulate at a rate of $2\mathrm{.}6\%,$ compounded quarterly, and payments of $$1200$ will be made at the end of every $3-$month period.

a$)$ How long will it take Lissa to repay her investor?

b$)$ How much interest will Lissa pay?