Two trains 100 m and 80 m in length are moving in the same direction, with speeds 51 m/s and 42 m/s respectively. In what time will they cross each other?
speed = $$51 m/s - 42 m/s = 9 m/s$$
speed = $$\frac{length of train1 + length of train2}{time}$$
9 =Â $$\frac{100 + 80}{time}$$
time =Â $$\frac{120}{9}$$Â
     20 seconds
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