9+(?)=−1
To know how much we need to add to the number 9 to get minus 1, we will count the number of steps between the numbers.
Also, we will pay attention to which direction we moved, if we moved right then the number is positive, if we moved left the number will be negative.
We will locate the number 9, and move left until we reach minus 1, where each step represents one whole number, as follows:
We discover that the number of steps is 10, and since we moved left, the number will be minus 10