Since there are 20 stations between Howrah and Burdwan and at each station it halts for 2 minutes.

Total Time that it have halted in all 20 station is 20*2=40 min.=2/3 hrs

Total time from Howrah to burdwan is 3 hrs in which total halt time is 2/3 hrs.

So total time in which the train has traveled is

3-2/3=7/3 hrs

Distance between Howrah and Burdwan is 19*5=115 km

so velocity is 120 / 7/3 = 115*3/7=345/7 km/hr