ahmedabad .........100 km...........baroda
(navjeevan express ) ........ (howrah- ahmedabad)
leaves at 6.30 am from ahmedabad ....... leaves at 7.00 am from baroda
speed=50 kmph ...... speed=40 kmph
now bcz traffic controller realize about problem at 7.30...firstly i have to find distance travelled by both train till 7.30
distance travelled by navjeevan express till 7.30=1h*50=50km
distance travelled by howrah ahmedabad till 7.30=1/2h*40=20 km
now we can see both trains moving towards each other so
remaining distance=100km-(50+20) km=30 km
relative speed=(50+40)kmph =90 kmph
so time remaining to avert head on collision=30km/90kmph=1/3 h=20 min