SOLUTION: The total length of train A and train B together is 777 m. Train B can cross an electric pole in 6 seconds. The respective ratio of speed of train A and train B is 7:8. How much ti

Question 935344: The total length of train A and train B together is 777 m. Train B can cross an electric pole in 6 seconds. The respective ratio of speed of train A and train B is 7:8. How much time will train A take to completely cross train B running in opposite direction (towards each other)?
Answer by TimothyLamb(4379) (Show Source): You can put this solution on YOUR website!
x = length of train A
y = length of train B
a = speed of train A
b = speed of train B
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x + y = 777
y = 777 - x
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speed of train B (meters/second):
s = d/t
b = y/6
---
speed of train A (meters/second):
a/b = 7/8
a = (7/8)b
a = (7/8)y/6
a = (7/48)y
---
relative speed of trains when running in opposite directions:
a + b = (7/48)y + y/6
a + b = (7/48)y + (8/48)y
a + b = (15/48)y
---
time for trains to cross each other when running in opposite directions:
s = d/t
t = d/s
t = (x + y)/(a + b)
t = 777/(15/48)y
t = 777*(48/15)/y
---
answer:
t = 2486.4/y seconds
where:
y = length of train B in meters
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