For this question let us take any time T>6 pm when the two trains meet.
the total distance between these two stations (D)=speed of either train * the time taken by that train=120*5 OR 100*6=600 km
now at time T, the train moving from agra to delhi must have covered=(T-5)*100 km
and The train moving from delhi to agra must have covered= (T-6)*120 km
As the two trains meet here therefore the sum of the distances covered by both trains should be equal to the total distance between Agra and Delhi.
So, (T-5)*100+(T-6)*120=600
Solving this we get T= 91/11 or 8:16 pm