Because the journey is 120km long, and the mortoristgoes less than 60km per hour. So in 2 hours, the motorist will go less than 120 km. So he needs more than 2 hours to go 120 km
A tree diagram can be drawn for a clearer understanding. The branches of the tails can be ignored since we are not concerned about that. To find the probability along the branches, we just have to multiply the probabilities of each branch, giving you an answer of 1/128