CAT Quantitative Aptitude Questions | CAT Number Systems - Remainders

The question is from CAT Remainders: Sum of digits. Sum of digits of a number is given and remainder when the number is divided by 11 is given. We need to find out the remainder when the number is divided by 33. Note: Sum of the digits of a number is nothing but the remainder when the number is divided by 9. Go on, try this out. Now, try to prove this.

Question 1: The sum of the digits of a number N is 23. The remainder when N is divided by 11 is 7. What is the remainder when N is divided by 33?

1. 7
2. 29
3. 16
4. 13

Video Explanation

Explanatory Answer

Method of solving this CAT Question from Number Theory - Remainders: Sum of the digits of a number is nothing but the remainder when the number is divided by 9. Go on, try this out. Now, try to prove this.

Sum of digits = 23
Remainder when N is divided by 9
N/9 = 5 {Remainder on division by 9 for any number is equal to the remainder of dividing the sum of the digits of the number by 9}
=> Remainder of N/3 = 2 {A number of the form 9k + 5 divided by 3 leaves a remainder 2}
N = 11k + 7
N = 3m + 2
11k + 7 => Possible numbers are 7, 18, 29, 40, 51
3m + 2 => Possible numbers are 2, 5, 8, 11, 14, 17, 20, 23, 26, 29

The number that is of the form 11k + 7 and 3m + 2 should be of the form 33b + 29. How did we arrive at this result?

The first natural number that satisfies both properties is 29.
Now, starting with 29, every 11th number is of the form 11k + 7, and every 3rd number is of the form 3m + 2.
So, starting from 29, every 33rd number should be on both lists (33 is the LCM of 11 and 3).
Or, any number of the form 33b + 29 will be both of the form 11K + 7 and 3m + 2, where b, k, m are natural numbers.
The remainder when the said number is divided by 33 is 29.

The question is "What is the remainder when N is divided by 33?"

Hence the answer is "29".

Choice B is the correct answer.