# 15 Secretly Funny People Working in the sum of digits of two digit number is 9

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20 For those of you who’ve never heard of the summation, it’s a tool that allows us to summarize the digits of a 2 digit number. This is useful if we want to do a little math and have a quick glance at what we’re doing.

But even without the summation, 9 is a nice way to write a number that is easy to remember. And the way it works is that the first digit is the number itself, then the second digit is the decimal in the number, and then the rest is the sum of the two digits.

So for example, if you wanted to count the number of cats in the house, you might write down, “the sum of the digits of the first five cats is 5”.

The same way that the sum of the digits of two digit numbers is 9, the sum of the digits of two digit numbers can be calculated. As long as you write it down on a piece of paper, you won’t forget it. The first digit of a number is always the number itself, the second digit is usually the decimal, and the rest of the number is the sum of the two digits.