A train covers 252 km at a uniform speed.  If the speed had been 15 km/h more, it would have taken 5 hours less for the same journey.  What is the speed of the train (in km/h)?

21

D = [\(\frac{S_1 × S_2}{Diff. (S_1\;and\;S_2)}\)] × Time (Diff.)

Let initial speed = S

Increased speed = S + 5

252 = \(\frac{S × (S + 15)}{15}\) × 5

S (S + 15) = 252 × 3

S (S + 15) = 21 × 36

⇒ S = 21 km/h