p$-$chart and c$-$chart. Using the appropriate control chart, determine two$-$sigma control limits for each case:

a$.$ An inspector found an average of $3\mathrm{.}9$ scratches in the exterior paint of each of the automobiles being prepared for shipment to dealers.

b$.$ Before shipping lawn mowers to dealers, an inspector attempts to start each mower and notes any that do not start on the first try. The lot size is $100$ mowers, and an average of $4$ did not start $(4$ percent$).$