A hypothetical deep earthquake occurred at time t$0$ and at a record depth of $720$ km directly beneath a seismic station. The operator at the station reports the arrival of a direct p wave $1$ minute and $20$ seconds after t$0.$ She also reports the arrival of a PcP wave at the same station $5$ minutes and $50$ seconds after the p arrival. A p wave is a P wave that travels upwards from the earthquake. A PcP wave is a P wave that travels down to the core$-$mantle boundary at a depth of $2820$ km$,$ reflects at the boundary and travels back up$.$

 What is the average P velocity of the mantle and crust above $720$ km$?$