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Question

An aeroplane left 30 minutes later then its scheduled time and in order to reach its destination 1500 km away in time it has to increase its speed by 250 km/h from its usual speed. Determine its usual speed

A
700 km/h
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B
750 km/h
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C
770 km/h
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D
800 km/h
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Solution

The correct option is B 750 km/h
Let x = usual speed

Usual time taken to reach destination:
t=1500x

Time to reach destination is now t - 0.5 (half hour), and speed is now x + 250
t0.5=1500x+250

1500x=(15000.5x)(x+250)

1500x=0.5x2+125x+375000

0.5x2+125x=375000

x2+250x=750000

x2+250x+15625=750000+15625

(x+125)2=765625x+125=875x=750

Usual speed =750 km/hr

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