Percentage increase in the rate of all rail season tickets to London = 4%
Price of the rail ticket from Cambridge to London increased by = 121.60
Let us assume the price before the increase in the fare = x
Then
4% * x = 121.60 dollars
(4/100) * x = 121.60 dollars
x/25 = 121.60 dollars
x = 25 * 121.60 dollars
= 3040 dollars
So the price of the ticket from Cambridge to London before the increase in price was 3040. I hope the procedure is simple enough for you to understand.