How fast would you able to calculate 105 X 104, without pen or paper or a calculator?
Let me demonstrate a method to compute the answer in your brain in less than 10 seconds!
If you are proficient with using a base number to calculate your squares or simple multiplications, this, then, is a breeze. First off, use 100 as your base number.
Simply add the deviations (either one) and .... multiply them (the deviations)!
105 X 104 = [104 + 5].[4 X5]
= 10920 #
Try it with other numbers nearer to 100 (either less or more). What do you think? With my own karat old brain, I can compute the answers in 5 seconds or less. Test yourself.