A parachutist drops freely from an airplane for 8s before the parachute opens out.  Then he descends with a net retardation of $2m{s}^{-2}$ reaching the ground with a velocity of $6m{s}^{-1}$.  The height from which he bails out of the aeroplane is $\left(g=10m{s}^{-2}\right)$