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J singh

· started a discussion

· 1 Months ago

sol (Question: 184418.0 )

explain logic, pls

Question:
A train X leaves Chandigarh 5 am and reaches Delhi at 9 am. Another train Y leaves Delhi at 7 am and reaches Chandigarh at 10 : 30 am. A for what time after 7 am do the both trains cross one another?
Options:
A) 48 min
B) 56 min
C) \(1\cfrac{1}{2}\) hrs
D) \(1\cfrac{1}{3}\) hrs
Solution:
Ans: (b)


Distance covered by first train till 7 am = 14 km

Remaining distance = 28 – 14 = 14 km

Time taken to cross after 7 am = \(\cfrac{14}{15}\) = 56 mins


Alternate sol:


Train X:

5 A.M to 9 AM =4 hours

Distance = x Km

Speed = x/4 Km per hour

Another Train Y :

7 A.M to 10.30 A.M = 7/2 hours

Distance = x Km

Speed = 2x/7 Km per hour

Let us assume that the two trains cross each other after h hours the X train leaves the station.

Distance covered by X at (5+h) A.M =hx/4 Km

Distance covered by another train(Y) at (5+h) A.M =(h-2)*2x/7 Km

Now hx/4 +(2hx-4x)/7 =x

Or h/4 +(2h-4)/7 =1

Or 7h+4(2h-4)=28

Or 15 h =44

h =44/15 hours =176 minutes

Time= 7.56 A.M

 So, required time= 56 minutes

Knowledge Expert

· commented

· 1 Months ago

Train X:
5 A.M to 9 AM =4 hours
Distance = x Km
Speed = x/4 Km per hour
Another Train Y :
7 A.M to 10.30 A.M = 7/2 hours
Distance = x Km
Speed = 2x/7 Km per hour
Let us assume that the two trains cross each other after h hours the X train leaves the station.
Distance covered by X at (5+h) A.M =hx/4 Km
Distance covered by another train(Y) at (5+h) A.M =(h-2)*2x/7 Km
Now hx/4 +(2hx-4x)/7 =x
Or h/4 +(2h-4)/7 =1
Or 7h+4(2h-4)=28
Or 15 h =44
h =44/15 hours =176 minutes
Time= 7.56 A.M
So, required time= 56 minutes
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