$2.$ We sometimes need to find how long it will take a sum of money $($or anything else$)$ to grow to some specified amount. Note that you should enter PV as a negative and FV as a positive.

$($a$)$ For example, if a company$$s sales are growing at a rate of $6\mathrm{.}24\%$ per year, how long will it take sales to double? Show your answer to $2$ decimals $($x$.$xx years$).$ Do not approximate using the $$rule of $72.$

$($b$)$ If you want an investment to triple in thirteen years, what interest rate must it earn? Show your answer to $2$ decimals $($x$.$xx$\%).$