Hi，I'm Betty.I come from Sydney.I'm a student.I eat breakfast at home.And then I go to school at 8：30.Classes start at 9：00.After school，I usually play sports with my friends.On the weekend，I often take a dancing class and play music.Sometimes I watch TV and read books.

    My favourite season is summer.Because Christmas is in summer.In Australia，summer is from December to February the next year.I usually go to the beach and swim on Christmas Day.What about you?

在我们生活和学习的周围，发生了太多意外的事。请你以“意外”为题眼，把题目补充完整。如：意外的收获、惊喜、发现……也可以直接以“意外”为题，写一件发生在你身上或你身边的事。

如图，在东西方向的海岸线上有A、B两个港口，甲货船从A港沿北偏东60°的方向以4海里/小时的速度出发，同时乙货船从B港沿西北方向出发，2小时后相遇在点P处，问乙货船每小时航行____海里．

已知 ，求 的值.