|
5/9. A way to solve this: Each can arrive between (I) 12:00 and 12:20, (II) 12:20 and 12:40, or (III) 12:40 and 13:00. If both arrive during I, or II, or III, they'll meet. If one arrives during I and the other during III, they won't meet. And if one arrives in II, and the other arrives in I or III, it's easy to see they have 50% odds of meeting. There are 9 possible arrival combinations, with 1/9 odds each, and summing gives 5/9 chances of a meeting. Another way of solving this: draw a square with the horizontal side representing the boy's arrival from 12:00 to 13:00 and the vertical side being the girl's arrival. Shade the area representing a meeting: it's a diagonal from bottom left to top right, 20 minutes wide. The shaded area is 5/9'ths of the total area. |
