The factors of 81 are 1, 3, 9, 27 and 81. If a number is divisible by another number, it is a factor of the given number. If there is a remainder, it cannot be a factor of the given number.
Every number is a factor of itself. The number one is a factor of every number. Thus, every number except 1 has at least two factors. Prime factors are numbers that are factors and prime at the same time. The number 81 has five factors but only one prime factor, the number 3. Factorization is very important in algebra.