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Question

A plane left 30 minutes later than the scheduled time. In order to reach the destination 1500 km away in time, it has to increase the speed by 250 km/hr from the usual speed. Find its usual speed.

A
800 km/hr
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B
680 km/hr
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C
750 km/hr
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D
1000 km/hr
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Solution

The correct option is C 750 km/hr
Let the usual speed of plane be x km/hr
and the increased speed of the plane be y km/hr.
y = (x + 250) km/hr
Distance = 1500 km [Given]
According to the question,
(Scheduled time) - (time taken at increased speed) = 30 minutes = 0.5 hours.

1500x1500y=121500x1500x+250=12 [Time=DistanceSpeed]1500x+3750001500xx(x+250)=12

x(x+250)=750000
x2+250x750000=0
x2+1000x750x750000=0
x(x+1000)750(x+1000)=0
(x750)(x+1000)=0
x=750 or x=1000
But speed cannot be negative.
The usual speed is 750 km/hr.

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