# Number theory

9 * 7 = 63 , 6 + 3 = 9 * 1

99 * 7 = 693 , 6 + 9 + 3 = 9 * 2

9 * 77 = 693 , 6 + 9 + 3 = 9 * 2

99999999 * 7 = x ,sum of digits in x = 9 * 8 =72

9 * 77777777 = x ,sum of digits in x = 9 * 8 =72

This is the easy way to get the sum of digits in product of numbers having 7 and 9 by multiplying 9 with greatest no. of 9s or 7s in the left hand side. Try it.

Note by Goutam Narayan
4 years, 6 months ago

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Nice!!!

- 4 years, 2 months ago

...how about other numbers? what techniques you know on how to find the sum of the digits of bigger number?

- 4 years, 6 months ago