What is the combined length of two trains when one, moving at $80\text{km/hr}$, crosses another, travelling in the same direction at $44\text{km/hr}$ in $40$ seconds?

When two trains cross each other, the total distance covered equals the sum of their individual lengths ($L_1+L_2$).
Since they are travelling in the same direction, we calculate their relative speed by subtracting the slower speed from the faster speed:

$$\text{Relative Speed}=80-44=36\text{km/hr}$$

Convert the relative speed into meters per second ($\text{m/s}$):

$$\text{Relative Speed}=36\times \frac{5}{18}=2\times 5=10\text{m/s}$$

Given time taken to completely cross each other is $t=40\text{seconds}$.

$$\text{Combined Length}=\text{Relative Speed}\times \text{Time}$$

$$\text{Combined Length}=10\text{m/s}\times 40\text{s}=400\text{m}$$