Fill the entire line with numbers.

1 2 3 4 5 6 7 8 9 10 11 ? 13 14 15 16 17 18 19 20 - forward counting.

Add 1 to the previous number.  

1 2 3 4 5 6 7 8 9 10 11 ? 13 14 15 16 17 18 19 20

? = 11 + 1 = 12

Or subtract 1 from the following one.  

1 2 3 4 5 6 7 8 9 10 11 ? 13 14 15 16 17 18 19 20

? = 13 - 1 = 12

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A pattern, an ornament, a sequence is the order of the arrangement of objects in such a way that a drawing can be continued by anyone. But to continue the pattern, you must first set the rule according to which the elements of the pattern are located.

Here is a row of flowers.

Are they arranged according to a certain scheme? What scheme is it? They alternate in color: purple, yellow, red. What will be the next flower after the red one? Purple. Then red and yellow. 

Here is the second sequence. 

These are triangles. They are the same color. That is, the drawing is not formed by color and shape. Do you see any pattern to continue this series? The first triangle is the smallest, the next one is the larger, the third one is the largest. Which triangle will you draw the fourth? The one that will be larger than the previous one. This is not an alternation, but an ascending arrangement. 

You already know a similar sequence. We call it a number line. The numbers in it are arranged in such a way that each subsequent one is greater than the previous one. Knowing this, you can continue the line from any place where it ends. 

What numbers are missing here? 11_12_13_14_15_?_?_18_19

There are no two numbers after the number 15. Let's define the first number. We know that it is larger than the previous one by one. Therefore, we increase fifteen by one: 15+1=16.

We can find the second number in two ways. The first way we found the number 16. Let's go along the second path. One of the properties of the number line is that any number can be obtained by reducing the number following it by one. Ahead of the unknown number stands 18. Let's subtract one from it: 18-1=17. Put the found numbers in the line: 11_12_13_14_15_16_17_18_19.

These rules will help you restore the order of the numbers.

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