Question

A number $n$ is said to be 'perfect' if the sum of all its divisors (excluding $n$ itself) is equal to $n$. An example of a perfect number is

• A. $9$
• B. $15$
• C. $21$
• D. $6$

Answer 1

Answer: (D)

Lets check each option one by one

A. $9$ divisor are $1$ and $3$.

$1+3=4\ne 9$ so not a perfect number.

B. $15$ divisors are $1,3,$ and $5$.

$1+3+5=9\ne 15$ so not a perfect number

C. $21$ divisors are 1, 3 and 7.

$1+3+7=11\ne 21$, so not a perfect number.

D. $6$ divisors are $1,2$ and $3$.

$1+2+3=6$ It is a perfect number

So correct answer is option D