The bus fare in a city is $$1\mathrm{.}75.$ People who use the bus have the option of purchasing a monthly coupon book for $$25\mathrm{.}00.$ With the coupon book, the fare is reduced to $$0\mathrm{.}50.$ a$.$ Express the total monthly cost to use the bus with a coupon book, f$,$ as a function of the number of times in the month the bus is used, x$.$ f $($x$)=$