On the advice of your uncle, you purchased $10$ shares of a well$-$established U$.$S$.-$based corporate stock for $$21\mathrm{.}5$ per share. After $1$ quarter, you received $$0\mathrm{.}25$ per share dividends each quarter for $2$ years. At that point, the stock price had gone down in a short$-$term recession, so you purchased $10$ more shares at $$15$ per share. The stock continued to pay $25c$ a share on all $20$ shares. After $3$ years $(12$ quarters$),$ you decided to sell the stock since it had increased in market value to $$24\mathrm{.}5$ per share. Make the following assumptions: $($a$)$ no commissions for the purchase or sale of the stock, $($b$)$ no government taxes on the dividends, and $($c$)$ quarterly compounding of the rate of return. What is the effective interest rate per year?

The effective interest rate per year is $\%.$