A) 39 km./hr.
B) 55.7 km./hr.
C) 22.3 km./hr.
D) 72.4 km./hr.
Correct Answer: A
Solution :
Speed of Rajdhani \[=x\]kmph. Speed of Express train \[=y\,kmph.\] According to the question, \[\frac{780}{y}-\frac{780}{x}=8\] ? (i) and, \[\frac{780}{x}-\frac{780}{y}=6\] ? (ii) Be equation \[(i)\times \frac{1}{2}+(ii)\] \[\frac{780}{x}-\frac{780}{2y}+\frac{780}{2y}-\frac{780}{2x}\] \[=6+4\] \[\Rightarrow \]\[\frac{780}{x}-\frac{780}{2x}=10\] \[\Rightarrow \]\[\frac{390}{x}=10\] \[\Rightarrow \]\[x=\frac{390}{10}=39\,kmph.\]
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