When counting forward, numbers are arranged in ascending order. To determine the unknown number, find out the difference between adjacent numbers. Add it to the number preceding the unknown one or subtract from the number that comes after the missing one.

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What is the order of numbers arranged in the line: 23 25 27 29 31 33 35 37 39 41?

The first digit is less than the second one: 23<25.

This rule works out for other numbers from the line too. We can conclude the numbers in the line increase from left to right. It is a forward sequence.

How to find the following number in the line? What number will come after 41? To find it out, we need to know the difference between the adjacent numbers. Let’s take any two adjacent numbers, for example 37 and 39, and subtract the less number from the greater one. The difference between these numbers equals 2.

To find the following number in the sequence, add 2 to the last known number. The last number that we know is 41. 41+2=43. It is the next number of this sequence.

Let’s find the number that comes before 23 in the line. To do this, subtract 2 from 23. We get 23-2=21. This number is on the left of 23. Now, the sequence looks like: 21 23 25 27 29 31 33 35 37 39 41 43.

Knowing the difference between adjacent numbers we can find the missing number in the sequence. It is the number between 53 and 73.

We can see the number of tens increases in each number by 1, and the number of ones remains the same. It means these numbers get greater from left to right. Let’s check it out and find the difference between adjacent numbers:

33-23=10 43-33=10 53-43=10 83-73=10

That’s right, the difference in all cases equals 10. We can add 10 to the previous number - 53, or subtract 10 from 73, which is the following number. In both cases, we get 63.

Now, we can fill in this number and complete the sequence.

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