Probability of meeting

[Guest]

A boy will arrive at a coffee shop randomly from 1 pm to 1.40 pm . He will stay for
20 mins and then leave . A girl will arrive at the coffee shop randomly from 1 pm
to 1.50 pm . She will stay for 10 mins and then leave . What is the probability that
they will meet there ?

[Guest]

The answer is 11 / 20 = 0.55