GMAT Questionbank | Number Properties Q2

The concept tested in this GMAT practice question is counting method and elementary number properties. A GMAT 650 to 700 level problem solving sample question.

Question 2: How many different positive integers exist between 10⁶ and 10⁷, the sum of whose digits is equal to 2?

1. 6
2. 7
3. 5
4. 8
5. 18

Video Explanation

Explanatory Answer

Method 1 to solve this GMAT Number Properties Question: Find the number of such integers existing for a lower power of 10 and extrapolate the results.

Between 10 and 100, that is 10¹ and 10², we have 2 numbers, 11 and 20.

Between 100 and 1000, that is 10² and 10³, we have 3 numbers, 101, 110 and 200.

Therefore, between 10⁶ and 10⁷, one will have 7 integers whose sum will be equal to 2.

Alternative approach

All numbers between 10⁶ and 10⁷ will be 7 digit numbers.
There are two possibilities if the sum of the digits has to be '2'.

Possibility 1: Two of the 7 digits are 1s and the remaining 5 are 0s.
The left most digit has to be one of the 1s. That leaves us with 6 places where the second 1 can appear.
So, a total of six 7-digit numbers comprising two 1s exist, sum of whose digits is '2'.

Possibility 2: One digit is 2 and the remaining are 0s.
The only possibility is 2000000.

Total count is the sum of the counts from these two possibilities = 6 + 1 = 7

Choice B is the correct answer.