Let T be the time taken and D is the distance For the case given in the problem, the distance covered is directly proportional to the time taken i.e D α T ⇒ D = ST where S is the constant speed D = 225 km and T = 4.5 hrs ∴ 225 = S x 4.5 S = \(\frac {225}{4.5}\) = 50km/hr When D = 150km D = ST 150 = 50 \(\times\) T T = \(\frac {150}{50}\) = 3 hrs So it will take 3 hrs