A train P starts from Meerut at 4 p.m. and reaches Ghaziabad at 5 p.m., while another train Q starts from Ghaziabad at 4 p.m. and reaches Meerut at 5:30 pm. At what time will the two trains cross each other?

speed of first train= $$\frac{D}{60}$$
speed of second train= $$\frac{D}{90}$$

suppose two train will meet after t minutes.

distance travelled by first train in t minutes=$$t\times \frac{D}{60}$$
distance travelled by second train in t minutes=$$t\times \frac{D}{90}$$
distance travelled by both trains= distance between stations
$$t\times \frac{D}{60}+t\times \frac{D}{90}$$=D

 D will cancel from both sides
$$\frac{D}{60}+ $$\frac{D}{90}$$=1
9 t+6 t=540
15 t= 540
t= $$\frac{540}{15}$$
t=36 minutes
 answer is 4:36 pm because first train start at 4 pm.