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

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

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