Two trains are each 50 m long moving parallel towards each other at speeds 10 m/s & 15 m/s respectively. At what time will they pass each other?

Let the trains moving opposite to each other
 The velocity of one train with respect to another train is=relative velocity  $=10-\left(-15\right)=25m/s$
 $$\begin{gathered} lengths of train \to l_1=l_2=50m \\ velocity of each train \to v_1=10m/s;v_2=15m/s \\ Total time to cross each other=t=\frac{l_1+l_2}{v_1+v_2}\Rightarrow t=\frac{50+50}{25}\Rightarrow t=4\sec \end{gathered}$$