Answer: light two incense sticks at the same time. The first incense is at both ends, and the other incense is only at one end. When the first incense sticks out (this is 3/4 of the total length), light the other end of the second incense, and the time from lighting to extinguishing at the other end is 15 minutes!
2. A manager has three daughters, and their ages add up to 13, which is equal to the manager's own age. A subordinate knows the manager's age, but still can't determine the age of the manager's three daughters. At this time, the manager said that only one daughter's hair was black, and then the subordinates knew the age of the manager's three daughters. What are the ages of the three daughters? Why?
A: The ages of the three girls should be 2, 2 and 9. Because only one child has black hair, that is, only she has grown up, and the other two are still very young, that is, less than 3 years old, with light hair. The manager should be at least 25 years old.
3. Three people went to a hotel and stayed in three rooms. The price of each room is $65,438+00, so they pay the boss $30. The next day, the boss thought that $25 was only enough for three rooms, so he asked my brother to return $5 to three guests. Unexpectedly, my brother was insatiable, and only returned 1 USD each, and secretly took it away by himself. Plus, my brother left 2 dollars, which makes a total of 29 dollars. But when the three of them paid $30, what about 1 dollar?
A: A * * handed over to 30 yuan includes 27 yuan (25 yuan gave it to the boss+younger brother embezzled 2 yuan) and everyone returned 1 yuan (***3 yuan). 2 yuan plus 27 is totally confusing.
Two blind people bought two pairs of black socks and two pairs of white socks respectively. Eight pairs of socks are made of the same material and size, and each pair of socks is connected with trademark paper. Two blind people accidentally mixed up eight pairs of socks. How can each of them get back two pairs of black socks and two pairs of white socks?
Answer: each pair of socks is unpacked, and each person takes a pair. There are no left or right socks. Finally, two pairs of black socks and two pairs of white socks were taken back.
5. A train leaves Los Angeles at a speed of 15km/h and goes straight to new york, while another train goes from new york to Los Angeles at a speed of 20km/h. If a bird leaves from Los Angeles at the same time with two trains at a speed of 30km/h, it meets another train and returns, flying back and forth in turn until the two trains meet. How long did it fly?
Answer: Convert the flight distance of birds into time calculation. Let the distance between LA and new york be a, the time when two trains meet is a/( 15+20)=a/25, and the flying speed of birds is 30, then the flying distance of birds is a/25*30=6/5a.
6. You have two cans, 50 red marbles and 50 blue marbles. Choose a jar at random and put a marble in the jar at random. How can you give red marbles the best chance? What is the exact probability of getting the red ball in your plan?
Answer: One jar contains a red ball, and the other jar contains 49 red balls and 50 blue balls. The probability is close to 75%.
This is the maximum probability that can be achieved.
In fact, it only needs a jar of 1. For everyone who wears black, he can see the black hat of N- 1.
, and therefore assume that they are
White. But after waiting for N- 1 times and no one hits himself, every black person can know that he is also black. So when the lights were turned off for the nth time, n people hit themselves.
12, two rings with radii of 1 and 2 respectively. The small circle goes around the big circle. How many times does the small circle turn by itself? If it is outside the big circle, how many times does the small circle turn by itself?
Answer: No matter inside or outside, the small circle turns twice. Small circles and big circles experience the same distance.
13, 1 Yuan, one bottle of soda, two empty bottles for one bottle. Q: You have 20 yuan money, how many bottles of soda can you drink at most?
Answer: 39 bottles, starting from the second bottle, equivalent to 1 yuan to buy 2 bottles.