| A constant distance from Chennai to Bengaluru is covered by express train at 100 km/h. If it returns to the same distance at 80 km/h, then the average speed during the whole journey is |
A) 90.20 km/h
B) 88.78 km/h
C) 88.98 km/h
D) 88.89 km/h
Correct Answer: D
Solution :
| Let the distance be x. |
| Then, \[\text{time}\,\,{{t}_{1}}=\frac{x}{100}h\] and \[\text{time}\,\,{{t}_{2}}=\frac{x}{80}h\] |
| \[\therefore \]\[\text{Average}\,\,\text{speed}=\frac{\text{Total}\,\,\text{distance}}{\text{Total}\,\,\text{time}}\] |
| \[=\frac{2x}{\frac{x}{100}+\frac{x}{80}}=\frac{800\times 2x}{18x}=\frac{800}{9}=88.89\,\,km/h\] |
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