A 450 m long freight train passes through a tunnel at a speed of 20 m/sec.
On the return journey it takes 45 sec longer for the train to pass completely
through the same tunnel when it travels at a lower speed of 15 m/sec. How
long is the tunnel?


  1. Speed=Distance/Time
    So Distance=Speed*Time
    Therefore we have L(Lenght of the tunnel)=V1 * t1 = V2 * t2
    There is a 45sec difference between t1 and t2, in other words                 t2-t1=45sec => t2= t1+45 (t2>t1 because it takes more time at a lower speed; V1>V2 => L/t1 > L/t2 => 1/t1 > 1/t2 => t1<t2: t1-t2=45 is not possible )
    So V1* t1 = V2* (t1+45) => t1 * (V1-V2)=V2 * 45 => t1= (V2* 45)/(V1-V2)= (15*45)/(20-15)=135m


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