每题最多想五分钟，从易到难。
1. At a train station: there is a south-bound train every 10 minutes; there is a north-bound train every 10 minutes. Every day you arrive at the train station at some random time, and you board the first train that arrives, whatever that is. After a year you found that 90% of time you get on the south-bound train. Why?

2. You are by yourself on an island, waiting to be rescued. Help will arrive on the 10th day (today is day 0). You have two types of pills: A and B, 10 tablets each. In order to survive to the next day, you need to eat one A and one B. However, you incidentally mixed the pills, and you cannot tell which is A, which is B. What do you do now?

3. There are 25 horses, each runs at different speed. You can race a maximum of 5 horses each time. What is the minimum number of races needed to determine the three fastest horses? (You need to determine which is No.1, which is No.2 and which is No.3).