Solve $72 - \left( {90} \right)$

Hint:
Here, we will use the rules of subtraction without using number lines. Using these rules will help us to know how to find the difference when a larger number is being subtracted from the smaller one. Subtraction is an arithmetic operation of removing a number of terms from a large group of terms.

Complete Step by step Solution:
Now, in order to subtract these numbers without using a number line, we should keep in mind the following steps of subtraction:
When a larger number has to be subtracted from a smaller number, we should learn the words ‘switch’ and ‘negate’.
Hence, this means that first of all, we should ‘switch’ the two numbers and do the normal subtraction, i.e. subtracting a smaller number from a larger one.
After subtracting, we should not forget to ‘negate’ the number obtained by attaching a ‘minus’ sign in front of the number.
In the given question, we can notice that a bigger number 90 is being subtracted from a smaller number 72.
Hence, we will first of all, ‘switch’ these numbers, thus we get:
$90 - 72 = 18$
Now, after subtracting and finding the result, we will ‘negate’ the result by attaching a minus sign, hence, we get:
$72 - \left( {90} \right) = - 18$

Therefore, $ - 18$ is the required answer.

Note:
We can use the number line to visualize how to subtract these numbers by thinking of the direction where we are required to move on the number line. But, since in this case, the numbers are bigger than usual, hence, using the number line would not be convenient and drawing it would be very time consuming and would make the question unnecessarily lengthy. Hence, we should know how to use these rules and applying them will help us to solve the questions correctly and in less time.