Original question: Hold out your left hand, count from your thumb, count to your little finger and count back, and repeat the count in turn like this. On which finger will the number 1000 fall? What about 2000? What about 20 13?

Solution: As can be seen from the figure, 1, 2, 3, 4, 5 is placed on five fingers, but from 6, there are only four numbers from ring finger to thumb, 6, 7, 8, 9, from 10, from index finger to little finger, only 165438. After removing the five numbers 3, 4 and 5, you start counting from the ring finger of your hand, that is, starting from 6, only four numbers are counted from right to left, and four numbers are counted from left to right, but counting from the index finger of your hand.

So when doing the problem, take 1000 as an example. After removing the numbers 1, 2, 3, 4 and 5, there is still the number 1000-5=995. Starting from 6, only four numbers are counted from right to left, and from left to right. 13, so we divide 995 by 4 to get 248+3, which means we count four from the ring finger to the left, and then count four from the index finger to the right for the second time. This process is repeated, we have to count to 248, because 248 is an even number, so we know that when we count to 248, we will return to the little finger of our hand, because there are still 3 left, so we have to start counting from it. Similarly, 2000 is on the index finger and 20 13 is on the little finger.

(1)( 1000-5)/4=248 The remainder 3 248 is even, so 1000 is on the index finger.

(May 2000) /4 = 498 The remainder 3 498 is an even number, so 2000 is on the index finger.

(2)(20 13-5)/4=502 502 is an even number with no remainder, so 20 13 is on the little finger.