To find out the missing number in the number line:

1. Determine the difference between given adjacent numbers.

2. Add this difference to the number that is placed before the unknown one.

3. Or subtract the difference from the number that follows the unknown digit.

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The numbers in the number line are placed in a particular order. But the difference between them doesn't always equal 1, as we used to think. It can be equal to 2.

Or 5:

And even 10: 

Why do we need to know the difference between adjacent numbers in the number line? To continue the sequence or complete if one digit is missing. As in this case: 

Let’s find the missing number:

1. We can determine the difference between adjacent numbers. To do this, let’s take two numbers that follow each other and subtract the less number from the greater one.  

45 - 43 = 2 

47 - 45 = 2  

49 - 47 = 2 

51 - 49 = 2 

57 - 55 = 2 

59 - 57 = 2  

61 - 59 = 2 

Now we know that the difference between them equals 2.  

2. We can find the missing digit by adding the difference to the previous number - 51.

51 + 2 = 53

3. If we subtract 2 from the number that is on the right of the missing digit, we will also find this unknown number.

55 - 2 = 53

4. Then we determine what number is missing and put it in the number line.

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