WORKED MATH ANSWER

HCF and LCM of 15 and 20

For 15 and 20, the highest common factor is 5 and the least common multiple is 60.

THE DIRECT ANSWER

HCF = 5. LCM = 60.

These two results answer different questions: the HCF is the largest shared divisor, while the LCM is the smallest positive shared multiple.

HIGHEST COMMON FACTOR5

5 divides both 15 and 20 exactly.

LEAST COMMON MULTIPLE60

60 is the first positive number both values divide into.

STEP 1 · LIST THE FACTORS

Factors of 15 and 20

Factors of 15: 1, 3, 5, 15
Factors of 20: 1, 2, 4, 5, 10, 20
Common factors: 1, 5

The common factors are the values that divide both numbers with no remainder. The greatest one is 5, so HCF(15, 20) = 5.

STEP 2 · USE PRIME FACTORS

Prime factorisation working

15 =3 × 5

20 =2² × 5

For the HCF, use only the shared primes at their smallest powers: 5 = 5. For the LCM, use every prime needed at its greatest power: 3 × 5 × 2² = 60.

15 × 20 = 300 = 5 × 60

TRY ANOTHER PAIR

Need to calculate different numbers?

Enter any valid pair or longer list in the full calculators. They update as you type and show their own working.

Open HCF Calculator Open LCM Calculator

What these answers mean

Use the HCF when you want the largest equal grouping size or to simplify a fraction. Use the LCM when you want the first time two repeating schedules meet, or a least common denominator for fractions.

This page is a worked answer for 15 and 20. It is not a replacement for checking your own question: use the calculators above for other inputs.