| A plane left 30 min later than the scheduled time and in order to reach the destination 1500 km away in time, it had to increase the speed by 250 km/h from the usual speed. Its usual speed is [NICL and GIC (AO) 2013] |
A) 720 km/h
B) 730 km/h
C) 740 km/h
D) 750 km/h
E) None of the above
Correct Answer: D
Solution :
| Usual speed of plane \[=x\,\,km/h\] |
| \[\therefore \]New speed \[=(x+250)\,km/h\] |
| \[\therefore \]\[\frac{1500}{x}-\frac{1500}{x+250}=\frac{30}{60}\] |
| \[\Rightarrow \]\[1500\left( \frac{x+250-x}{x(x+250)} \right)=\frac{1}{2}\] |
| \[\Rightarrow \]\[x\,\,(x+250)=1500\times 500=750000\] |
| \[\Rightarrow \]\[x\,\,(x+250)=750\,\,(750+250)\] |
| \[\Rightarrow \]\[x=750km/h\] |
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