given train speed is 60km/hr---> 1km/min
total length of train--> (60*20)+5(gaps)
therefore, total length to travel--> 1km+125 mtrs
clearly, it takes exactly 1 min for train to traverse the length of the tunnel but it has to travel its full length i.e., 125 mtrs to come outside of the tunnel completely.
Now since train is travelling at 1km/min-->1000mtrs/min-->1000mtrs/60 seconds
it takes 7.5 seconds to travel 125mtrs, 7.5 seconds-->1/8th of a minute
so train takes--> 1+1/8 minutes to cross tunnel completely