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An aeroplane takes 1hr less for a journey of 1200km. If its speed is increased by 100km/hr from its usual speed.Find its usual speed?

Let original speed be x.

time = distance/speed

original time = 1200 / x

new time = 1200 / x+100

1200 / x - 1200 / x + 100 = 1

1200  (1 / x - 1 / x + 100)=1

1200 (x + 100 - x / x* +100x) =1                   here x* is x square.

120000 = x * + 100x

x * + 100x -120000 =0

therefore by solving equation  

x = 300 km/hr


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