$7\mathrm{.}8$ An earthmoving contractor encounters a trap$-$rock formation at shallow depth in a cut. Seismographic tests indicate a seismic velocity of $6,400fps$ for the material On this basis, it is proposed to rip the rock with a $370-$hp crawler tractor. Estimate the production in bcy$,$ for full$-$time ripping, with efficiency based on a $45-$min hour. The ripper is equipped with a single shank and the ripping conditions are average $($i$.$e$.,$ intermediate between extreme conditions$).$ The normal O&O cost per hour, including operator, for a $370-$hp tractor with ripper is $$168.$ This is classified as heavy ripping. Using your estimate of hourly production, estimate the ripping unit$-$production cost in dollars per bcy$.$