What is the prime factorization of 15?

15 = 3 × 5. Both numbers are prime, so the factor tree is complete after one split.

What are the factor pairs of 15?

The positive factor pairs of 15 are (1, 15) and (3, 5).

Factors of 15 - Factor Pairs & Prime Factorization

Q: What are the factors of 15?

The factors of 15 are 1, 3, 5 and 15. It has four factors.

Q: Is 15 a prime or composite number?

Composite. It has four factors, and it is divisible by both 3 and 5.

What Are the Factors of 15?

Imagine 15 chairs for a school event. You can set them in 1 long row of 15, in 3 rows of 5, or in 5 rows of 3. Each of those layouts is a factor. A factor is a whole number that divides another number exactly, with nothing left over.

The factors of 15 are 1, 3, 5 and 15. That is four factors.

Factor Pairs of 15

Look for pairs that multiply to 15 and work from the outside in.

Handwritten notebook page titled Factors of 15 showing 1 × 15 = 15 and 3 × 5 = 15, with the four factors 1, 3, 5 and 15 circled in red and the note 4 factors — The two pairs of 15, then all four factors circled.

Start with 1 × 15. Every number has this pair.
Try 2. Since 15 ÷ 2 leaves 1, skip it.
Try 3. Since 15 ÷ 3 = 5, write 3 × 5.
Try 4. It leaves a remainder, and the next number, 5, is already written, so you are done.

Prime Factorization of 15

Prime factors show what 15 is built from. Fifteen splits into 3 and 5, and both are already prime.

Handwritten factor tree of 15 splitting into red-circled 3 and 5, with 15 = 3 × 5 written below in green — Factor tree of 15. One split gives two primes.

So 15 = 3 × 5. Using the shortcut, (1 + 1) × (1 + 1) = 4 factors, which matches the list.

Divisibility check for 15

A number divides by 15 when it divides by both 3 and 5. So it must end in 0 or 5, and its digits must add to a multiple of 3. For 135, the last digit is 5 and 1 + 3 + 5 = 9, so 135 is a multiple of 15.

Is 15 a Prime or Composite Number?

Composite. It has four factors rather than two. The sum of all its factors is 1 + 3 + 5 + 15 = 24.

Real-Life Uses of the Factors of 15

Minutes. A quarter of an hour is 15 minutes, so 15 divides the hour into four equal parts.
Tennis scoring. Points go 15, 30, 40, which is a pattern of 15s.
Sharing. Fifteen cards deal evenly to 3 or 5 players.

Common Factors of 15 With Other Numbers

The factors of 20 are 1, 2, 4, 5, 10 and 20, so 15 and 20 share 1 and 5. The GCF is 5, which turns 15/20 into 3/4.

Quick Practice

Which of these are factors of 15: 3, 4 or 5? That is 3 and 5. Now try 30 and check it with the calculator.

Factors of 15