How to Multiply by 9, 99, 999, 9999 Faster

Before I explain the previous post about multiplication, I would like to discuss first multiplication by 9, 99, 999, 9999 or any number whose digits are all 9. For example, how do we multiply 87 × 99? To multiply by numbers whose digits are all 9, we just add 1 to 9, 99, 999, 9999 and so on and […]