80 railway wagons all of same mass  $5\times {10}^{3}kg$ are pulled by an engine with a force of  $4\times {10}^{5}N.$ The tension in the coupling between 30^th and 31^st wagon from the engine is

Total mass of 80 wagons = $80\times 5\times {10}^{-3}=4\times {10}^{5}kg$

Acceleration,  $a=\frac{F}{M}=\frac{4\times {10}^5}{4\times {10}^5}=m{s}^{-2}$

Tension in the coupling between 30^th and 31^st wagon will be due to mass of remaining 50 wagons. Now, mass of remaining 50 wagons.

$m=50\times 5\times {10}^{3}kg=25\times {10}^{4}kg$

$\therefore \mathrm{Re}quiredtension,T=mg=125\times {10}^4\times 1$

$=25\times {10}^{4}N$