You can create a complete portfolio comprised of risk$-$free treasury bills, which are expected to earn

$5\%,$ and a risky mutual fund with an expected return of $18\%.$ Your brokerage firm will also let you borrow funds up to $50\%$ of the value of your portfolio at the risk$-$free rate. If you wish to generate a $20\%$ return on your complete portfolio how much should you invest in the risky mutual fund?

a$.0\%$ b$.50\%$ c$.100\%$ d$.115\%$