Is 155 a prime number?
It is possible to find out using mathematical methods whether a given integer is a prime number or not.
For 155, the answer is: No, 155 is not a prime number.
The list of all positive divisors (i.e., the list of all integers that divide 155) is as follows: 1, 5, 31, 155.
For 155 to be a prime number, it would have been required that 155 has only two divisors, i.e., itself and 1.
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Actually, one can immediately see that 155 cannot be prime, because 5 is one of its divisors: indeed, a number ending with 0 or 5 has necessarily 5 among its divisors. The last digit of 155 is 5, so it is divisible by 5 and is therefore not prime.
As a consequence:
For 155 to be a prime number, it would have been required that 155 has only two divisors, i.e., itself and 1.
However, 155 is a semiprime (also called biprime or 2-almost-prime), because it is the product of a two non-necessarily distinct prime numbers. Indeed, 155 = 5 x 31, where 5 and 31 are both prime numbers.
Is 155 a deficient number?
Yes, 155 is a deficient number, that is to say 155 is a natural number that is strictly larger than the sum of its proper divisors, i.e., the divisors of 155 without 155 itself (that is 1 + 5 + 31 = 37).