In order to accumulate $$1,500,000$ over $20$ years, how much would you have to invest at the beginning of every three months into a fund earning $7\mathrm{.}6\%$ compounded quarterly? a$$3,594\mathrm{.}26$ b$$7,973\mathrm{.}96$ c$$8,125\mathrm{.}47$ d$$13,359\mathrm{.}69$