Thread: probability problem..pls help

there are 2 persons A and B. they decide to meet in a park between 10 am to 11 am. it was decided that whosoever comes first will wait for the other person for exactly 20 minutes. what is the probability that they do not meet? Please provide multiple methods of solving. Thanks.

This is a fairly classic problem...one of the solutions involves drawing a square of side length 1 on an xy-plane (say, with coordinates (0,0), (1,0), (1,1), (0,1)). We label the axes so that the x-axis corresponds to A's arrival time, and the y-axis corresponds to B's arrival time. Here, (0,0) corresponds to both A and B arriving at 10:00, (1/2, 1/2) corresponds to both A and B arriving at 10:30, and so on.

The probability that A and B do meet is given by a certain geometric region S. Find the area of S. The probability that they do not meet is simply 1-S.