Multiply a 2-digit Number by 99

I have left a few more weeks to complete all my term-papers and to face my final exams. Having stress and anxiety within me, one of my ways to destress myself is to play with numbers. So I have decided to share how to get the answer of a 2-digit number (mn) mulitplying by 99 without a calculator. (Does not apply to 99 x 90 (to 99) )

99 x (mn) = abcd (where abcd stands for each single digit)

9 x (m+1) = ac
9 x (n) = bd

For example, 99 x 25:

we take 9 x (2+1) = 9 x 3 = 27.
Therefore, the places where "a" and "c" stand will be replaced by "2" and "7", which becomes 2b7d.

Next, we take 9 x 5 = 45, which takes "b" and "d", so it will be a4c5

Combining both, we will have 99 x 25 = 2475.

More examples,

99 x 17:
9 x (1+1) = 9 x 2 = 18 (1b8d)
9 x ( 7) = 63 (a6c3)
therefore, 99 x 17 = 1683

How fast can you solve the following?
a) 99 x 37
b) 99 x 68
c) 99 x 78
d) 99 x 24
e) 99 x 59
f) 99 x 55
g) 99 x 19

What other patterns can you derive from (a), (f), and (g)?
Look out for the answer in the next posting...


Acknowledgement: I thank my Pri 5 teacher, Mrs Neo, who taught me this, which I remember up to this day.