Equation of trajectory of a projectile is given by y=8x−x25 where x and y are in metre. If g=10 m/s2, time of flight is

General equation of trajectory is y=xtanα−g x22u2cos2αcomparing this equation with given equation tanα=8 and g sec2α2u2=15⇒g1+tan2α2u2=15∴ u2=5×10×1+822⇒u=565 m/s∴ T=2usinθg=2×56510×81+82sec=8 sec