Question

why is 23 deficient?

Answer 1

Take the sum of the positive divisors of a number excluding the number itself but including 1

If that sum equals the original number, it is called a perfect number.
e.g.
28 has factors of 1,2,4,7,14
1+2+4+7+14 = 28 , so 28 is a perfect number

if the sum exceeds the number , the number is considered abundant.
e.g. factors of 12 are 1,2,3,4,6
1+2+3+4+6 = 16
12 is abundant.

So that leaves us with a defincient number where the sum of the factors is less than the number.
23 is prime, so the sum as defined above is 1
making 23 deficient.