Let's assume that a ticket from G to J costs the same regardless if it is a direct ticket or an additional stop (H or I).
Then, if we buy all three combinations of tickets, we get:
- 3 tickets from G to J
- 1 ticket from G to H and 1 ticket from H to J
- 1 ticket from G to I and 1 ticket from I to J
So, what we actually get is:
5 tickets from G to J for the price of £1+£1.20+£1.30=£3.50
Hence one ticket from G to J costs £0.70.