You don't need to say a thing Part 4

Logic Level 1

Think of a 2 digit positive integer. Multiply it by 3 and add 6. Then multiply it by 3 and add 13. Find the sum of the digits of the resultant number. And again, find the sum of digits of the last resultant number. I know what number you have left. What must it be?


The answer is 4.

Chung Kevin by Chung Kevin , 45, Australia

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5 solutions

Cantdo Math
Apr 14, 2020

take modulo 9 simply.

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Adrian Ma
Apr 4, 2020

For these types of questions you can just substitute in any number and find the answer. e.g 10 3=30. 30+6=36. 36 3=108. 108+13=121. 1+2+1=4. 4=4

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Alan Yan
Aug 10, 2015

Denote the number by x. After multiplying by three and adding 6, the number will be 3x+6. After multiplying by three and adding 13 the number will be 3x+31. The sum of the digits is equivalent to the number in modulo 9. Therefore, the sum of the digits is 31(modulo 9) which is equivalent to 4.

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Vijay K
Aug 10, 2015

Multiples of 9 when added to 13 end up with the result whose sum will be 4 (1 + 3)

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Tariq Khan
Aug 9, 2015

12 X 3=36 +6=42 x 3=126+13=139, 1+3+9=13, 1+3=4

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