I can multiply any three-digit number by 1667 in my head, usually in less than 10 seconds.
Also, to make it even more impressive, I come up with the digits of the result in proper order (not in reverse!)
How can I do this?
Divide your number by 6. This gives the first 2 or 3 digits of the answer.
Multiply the remainder by 5.
Put this remainder by 5 in front of your original number and divide by 3. This gives the rest of the answer.
Example:
If we are to multiply 1667 by 983.
983 divided by 6 is equal to 163 remainder is 5.
Multiplying 5x5 we get 25.
Putting this in front of the number and dividing by 3, we obtain:
25983/3 = 8661
Therefore, the required product is 1638661.
Edited on May 17, 2023, 11:14 pm
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