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The digital sum of a number is the sum of its digits.

This topic has 2 expert replies and 0 member replies

The digital sum of a number is the sum of its digits.

Post Thu Nov 02, 2017 8:36 am
The digital sum of a number is the sum of its digits. For how many of the positive integers 24-125 inclusive is the digital sum a multiple of 7?

A 7
B 8
C 14
D 16
E 20

The OA is C.

Do I have to make the entire list? Is there another way to solve this PS question? Experts, I want to see you explanation for this exercise. Thanks.

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GMAT/MBA Expert

Post Fri Nov 03, 2017 1:55 pm
Hi Vincen,

Based on the answer choices, you know that there are at least 7 numbers that fit the given description, but no more than 20. Thus, listing them all out shouldn't be too difficult. You might also find it useful to organize your numbers by 'group'

We're restricted to the numbers 24 to 125, inclusive. We're asked to find the numbers whose digits add up to a MULTIPLE of 7:

Numbers with digits that add up to 7:
25 and 52
34 and 43
61
70
106
115
124

Numbers with digits that add up to 14:
59 and 95
68 and 86
77

Total numbers = 9+5 = 14

Final Answer: C

GMAT assassins aren't born, they're made,
Rich

_________________
Contact Rich at Rich.C@empowergmat.com

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GMAT/MBA Expert

Post Thu Nov 02, 2017 9:16 am
Vincen wrote:
The digital sum of a number is the sum of its digits. For how many of the positive integers 24-125 inclusive is the digital sum a multiple of 7?

A 7
B 8
C 14
D 16
E 20
Listing and counting is a fast approach.
25
34
43
52
61
70
106
115

Answer: B

Cheers,
Brent

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Brent Hanneson – Founder of GMATPrepNow.com
Use our video course along with Beat The GMAT's free 60-Day Study Guide

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