The fifth unit of a $25-$unit job took $\backslash (14\mathrm{.}5\backslash )$ hours to complete. If a $90$ percent learning curve is appropriate: $\backslash ($ a $\backslash ).$ How long should it take to complete the last unit? b$.$ How long should it take to complete the $10$ th unit? c$.$ Estimate the average time per unit for the $25$ units.