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.