SOLUTION: A train running at a uniform speed passes a bridge 275m long in 15 seconds and another bridge 425m long in 21 seconds. Find : 1. the length of the train: 2. the speed of the trai

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Question 600459: A train running at a uniform speed passes a bridge 275m long in 15 seconds and another bridge 425m long in 21 seconds. Find :
1. the length of the train:
2. the speed of the train in km/hr.
Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
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A train running at a uniform speed passes a bridge 275m long in 15 seconds and another bridge 425m long in 21 seconds.
Find
:
1. the length of the train:
2. the speed of the train in km/hr.
:
Let t = length of the train in meters
Let s = the speed of the train in meters/sec
:
write a speed equation for each bridge: Speed = dist/time
:
s = %28%28t%2B275%29%29%2F15
and
s = %28%28t%2B425%29%29%2F21
therefore
%28%28t%2B275%29%29%2F15 = %28%28t%2B425%29%29%2F21
Cross multiply
21(t+275) = 15(t+425)
:
21t + 5775 = 15t + 6375
:
21t - 15t = 6375 - 5775
6t = 600
t = 100 meters is the length of the train
:
Find the speed
s = %28%28100%2B275%29%29%2F15
s = 375%2F15
s = 25 meter/sec
:
Convert to km/hr:
S = %28%283600%2A25%29%29%2F1000
S = 90 km/hr