Multiply two numbers when a round number sits halfway between them.
Example
{% scaleimage l11_example.png float_right %}
Step 1
Identify the round number sitting equally between the two numbers
{% scaleimage l11_step1.png %}
Halfway
Step 2
Square the round number
{% scaleimage l11_step2.png %}
Step 3
Square the difference between the round number and the two numbers
{% scaleimage l11_step3.png %}
Step 4
Subtract step 3 from step 2
4900 - 9 = 4891