There is a sequence of numbers as follows with which the number is located in a line under a certain rule.
What is the 20th number?
1，2，2，4，3，6，4，8，5，10，……
Answer
20
Solution
This sequence of numbers is combined by two types of sequence of numbers and arranged by turns.
One is "1, 2, 3, 4, 5…", another is "2, 4, 6, 8, 10,…".
Writing out to the 20th, "1, 2, 2, 4, 3, 6, 4, 8, 5, 10, 6, 12, 7, 14, 8, 16, 9, 18, 19, 20".
Then you can find the 20th number is 20.
