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