Question 1036205
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Two trains 100 meters and 120 meters long are running in the same directions with speed of 72 km/hr and 54 km./hr. 
In how much time will the first train cross the second?


<U>Answer</U>. &nbsp;{{{(100+120)/(20-15)}}} = {{{220/5}}} = 44 seconds.


<U>Solution</U>


<pre>
What is &nbsp;20&nbsp; in the denominator in the <U>Answer</U>? &nbsp;It is &nbsp;20 meters per second, 
   which is the speed of the first train, &nbsp;72&nbsp; kilometers per hour, converted to meters per second.


What is &nbsp;15&nbsp; in the denominator in the <U>Answer</U>? &nbsp;It is &nbsp;15 meters per second, 
   which is the speed of the second train, &nbsp;54&nbsp; kilometers per hour, converted to meters per second.


What is the difference &nbsp;(20-15)&nbsp; in the denominator? &nbsp;It is the relative speed of the faster train to slower train
   moving on parallel tracks in the same direction. 


What is the sum &nbsp;(100+120)&nbsp; in the numerator? &nbsp;It is the sum of the lengths of the two trains. 


Why we divide &nbsp;(100+120)&nbsp; by &nbsp;(20-15)? &nbsp;It is the time required for the faster train to pass completely the slower train. 
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See the lesson <A HREF=https://www.algebra.com/algebra/homework/word/travel/A-train-passing-another-train.lesson>A train passing another train</A> in this site.