Yesterday, my son (7-year old) taught me two tricks. I knew one of them.
The first one should be simple, any two digit number times 11 could be done quickly by separating two digits and put the sum of both in the middle.
For example: 23 * 11 = 2 (2+3) 3 = 253
If the sum is bigger than 10 - carry over to the highest.
For example: 56 * 11 = 5 (5 + 6) 6 = (5+1) 1 6 = 616
And here is the second one:
The square of the two-digit number which ends at 5.
For example, 75*75 = 7(7+1)25 which is 5625
The steps are:
- Take first digit and multiple its next - resulting the left-most digits of the answer
- Append “25” to the above.
Let’s take another example:
65^2 = 6(6+1)25 =4225
I am so proud of my son!
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