Two stations A and B are $210\text{km}$ apart on a straight line. One train starts from A at 7 a.m. and travels towards B at a speed of $35\text{km/h}$. Another train starts from B at 8 a.m. and travels towards A at a speed of $40\text{km/h}$. At what time will they meet?

Total distance between A and B = $210\text{km}$.
The first train starts from A at 7 a.m. and travels towards B at $35\text{km/h}$.
Distance covered by the first train in $1\text{hour}$ (from 7 a.m. to 8 a.m.):

$$\text{Distance}=\text{Speed}\times \text{Time}=35\text{km/h}\times 1\text{h}=35\text{km}$$

Remaining distance between the two stations at 8 a.m.:

$$\text{Remaining Distance}=210\text{km}-35\text{km}=175\text{km}$$

At 8 a.m., the second train also starts from B towards A at a speed of $40\text{km/h}$.
Since both trains are moving in opposite directions, their relative speed is:

$$\text{Relative Speed}=35\text{km/h}+40\text{km/h}=75\text{km/h}$$

Time taken by both trains to meet after 8 a.m.:

$$\text{Time}=\frac{\text{Remaining Distance}}{\text{Relative Speed}}=\frac{175}{75}\text{hours}=\frac{7}{3}\text{hours}$$

$$\frac{7}{3}\text{hours}=2\text{hours}+\frac{1}{3}\text{hours}=2\text{hours }20\text{minutes}$$

Meeting time = $8\text{:00 a.m.}+2\text{hours }20\text{minutes}=10\text{:20 a.m.}$