It takes t to cross the frozen soil section.

According to the title, the time to pass through the unfrozen section is 2. 1 times that of the frozen section.

Then the time to pass through the unfrozen road section is 2.1t.

Distance = speed x time

The frozen soil section distance = 100t km.

Distance of unfrozen section = 120x2. 1t = 252tkm.

Then the total distance s= 100t+252t=352t km.

Third question

Suppose it takes u hours to cross the frozen soil section.

It takes u-0.5 hours to pass through the unfrozen section.

The frozen soil section distance = 100u km.

Distance of unfrozen section =120 x (u-0.5) = (120 u-60) km.

The length of the railway is s =100 u+120 u-60 = (220 u-60) km.

Distance between frozen section and unfrozen section =120u+60-100u = (20u-60) km.

I hope you can adopt.