Time = distance/speed

Given, areoplane takes 1 hour less for a journey of 1200 km if its speed is increased by 100 km / hr from its usual speed.

Let the usual speed be ‘a’.

⇒ 1200 ×(a + 100 –a) = a² + 100a

⇒ a² + 100a – 120000 = 0

⇒ a² + 400a – 300a – 120000 = 0

⇒ a(a + 400) – 300(a + 400) = 0

⇒ (a + 400)(a – 300) = 0

⇒ a = 300 km/hr