Time = distance/speed

Given, aeroplane left 50 minutes later than its schedule time, and in order to reach the destination, 1250 km away, in time, it had to increase its speed to 250 km / hr from its usual speed.

Let the usual speed be ‘a’.

⇒ 6 × 1250 × (a + 250 –a) = 5(a² + 250a)

⇒ a² + 250a – 375000 = 0

⇒ a² + 750a – 500a – 375000 = 0

⇒ a(a + 750) – 500(a + 750) = 0

⇒ (a + 750)(a – 500) = 0

⇒ a = 500 km/hr