If a person travels by car at 36 km/hr, he reaches his destination on time. He covers half the journey in 4/5 of the time. What speed should he maintain for the remaining part to reach on time?

The required speed is 90 km/hr.

Let total time on time be T, so half the distance took 4/5 T.

The remaining half must be done in T - 4/5 T = 1/5 T.

At 36 km/hr the full distance D = 36T, so half is 18T.

Required speed = 18T / (T/5) = 18 x 5 = 90.

Half the distance in one-fifth of the time needs five times the half-speed.