Look at the sketch on page 342. Example 2 page 342 and 343 gives some more examples which apply straight-forward calculations of areas to get the desired probabilities. Example 3 page 343 . Probability of two people meeting.
Person waits five minutes and leaves.
Find probability that they meet. Draw sketches for each problem. Do this like Example 1, problem 1 of this section. Do this like Example 2, a,b,c of this section 6. Do this by first writing out what each of these sets means.