What plus five equals nine?
Here we have an addition calculation. But the missing number is at the start. We start with a number we don’t know what it is yet. Then, we add five to that number. And we get a total of nine. What number is it that if we add five to it, we’ll get nine?
Here are nine cubes. Here is the five part of the number nine. These are the five that we add on to our mystery number to make nine. But how many are in the other part? What number pairs with five to make nine? One, two, three, four. Four and five are a pair that make nine. We know five plus five equals 10. And so if we add one less than this, four plus five equals nine.
The missing number is four.