In Spring $2014,$ some car dealers offered a cash$-$back allowance of $$4000$ or $0\%$ financing for $72$ months on a Chevrolet Silverado.

$($a$)$ Determine the monthly payments on a Chevrolet Silverado if a buyer chooses the $0\%$ financing option and needs to finance $$30,000$ for $72$ months $($interest is compounded monthly$).$

$($b$)$ Determine the payments on a Chevrolet Silverado if a buyer chooses the cash$-$back option and now needs to finance only $$26,000.$ At the time, it was possible to get a new car loan at $3\mathrm{.}18\%$ for $48$ months, compounded monthly. Find the total amount of the payments