The difference between a number and 9 is 27. Find that number

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The difference between a number and 9 is 27. Find that number

The phrase a number means an arbitrary variable, let's call it x:
x

The difference between a number and 9
x - 9

The word is means equal to, so we set x - 9 equal to 27:
x - 9 = 27

To solve this equation for x, we type it in our math engine and we get:
x = 36