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Surinder Wasan

· started a discussion

· 1 Months ago

4th question is technically wrong (Question:652340.0)

it should take more time to cross a cyclist going along , or it should have been mentioned in question that they are coming towards each other

Question:
If a train takes 1.75 sec to cross a telegraph post and 1.5 sec to overtake a cyclist racing along 10 m per second, then the length of the train is:
Options:
A) 135 m
B) 125 m
C) 115 m
D) 105 m
Solution:
Ans: (d)

Suppose speed of train = x m/s

or length of train = l m

Time taken by trains to cross a telegraph cost 

= \(\cfrac{Length\ of\ train}{Speed\ of\ train }\)

\(\therefore\ 1.75=\cfrac{l}{1.75}\)

or   \(x=\cfrac{l}{1.75}\)     ........ (i)

\(\therefore\ 1.5=\cfrac{l}{x-10}\)

or  \(x-10=\cfrac{l}{1.5}\) ......... (ii)

Using (i) and (ii)

\(\cfrac{l}{1.75}-10=\cfrac{l}{1.5}\)

or  \(\cfrac{l}{1.5}-\cfrac{l}{1.75}=10\)

or 0.25 l = 10 \(\times\) 1.5 \(\times\) 1.75

 \(\therefore\ \cfrac{10\times1.5\times1.75}{0.25}=105\) m

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