The prime factors of 40 are 2 x 2 x 2 x 5. Prime factors are prime numbers, which are only divisible by 1 and themselves. The only even prime number is 2, and it is a factor of all even numbers.
Because 40 is even, determining its prime factors begins with dividing by 2, leaving the factors of 2 and 20. Since 20 is even, it is not prime, so it is necessary to divide by 2 again, leaving factors of 2, 2 and 10. A final division by 2 shows the prime factors of three 2s and a 5. Every number has its own unique set of prime factors.
Alternatively, one can simply begin by dividing the original number in any way that seems easiest, and then divide the subsequent divisors into smaller numbers, until only prime numbers remain. Using this method, one can calculate the prime factorization of 40 in the following way: 40 = 8 x 5 = 4 x 2 x 5 = 2 x 2 x 2 x 5. Note that either method arrives at the same end result for all whole numbers.