Why is 11 a deficient number?

Answer 1

11 is divisible by 11.
11 times 2 is 22.
11 < 22
11 is deficient.

13 is divisible by 4 and 13.
The sum of 4 and 13 is 17.
13 times 2 is 26.
17 < 26
13 is deficient.

and so on...

You just see what goes into the number (in this example, we use the numbers 11 and 13), then add them all together. Then you multiply the number you're investigating with 2, and if the result of this is greater than the sum, it is deficient.

Let's check 12 (which is not deficient):

12 is divisible by 2, 3, 4, 6, and 12
The sum of 2, 3, 4, 6, and 12 is 27
12 times 2 is 24
24 < 27

The sum is greater, so 12 is not deficient.