Really cool math tricks

The fantastic 11 rule
There is a great likelihood that many people know the 10 rule. That is, when you multiply a number by 10, you add a 0 at the end of the number. However, the 11 rule may be unknown to many. It is so easy to comprehend.
- As you multiply any two digit number by 11
- Let us take 36 for this example (3-6) and notice the gap in between them.
- Then, sum up the two digits, 3 and 6 together
- Put the result, 9 in the gap in between.
- That is simply done 39 x 11=3 96

However, there is one tricky thing that you should remember, if the result after you have added the two digits is more than9, put only the digit representing the "ones" in the gap. For example:
74x11€7_4€.4 +7=11€
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