What is the highest common factor of 24 and 90?

To find the highest common factor of 24 and 90 we do this in three steps:
1. Find the prime factorisation of the two numbers. This means rewrite each given number as a product of only prime numbers. The prime numbers being 2, 3, 5, 7, 11, 13, 17….. 
24 = 2 * 2 * 2 * 3 = 2^3 * 3
This means that when multiplying 2 x 2 x 2 x 3 you obtain 24. 
90 = 2 * 3 * 3 * 5 = 2 * 3^2 * 5
This means that when multiplying 2 x 3 x 3 x 5 you obtain 90. 
2. Find the prime factors that divide both numbers 
24 = 2 * 2 * 2 * 3 
90 = 2 * 3 * 3 * 5 
Both 24 and 90 are divisible by the numbers 2 and 3 (write down the prime numbers that appear in both expressions) 
3. Multiply the prime factors common to both numbers 
Since 2 and 3 appeared in both prime factorisations, they are known as the common prime factors”. To calculate the highest common factor between 24 and 90 you need to multiply their common prime factors. 
Highest common factor between 24 and 90 = 2 * 3 = 6
And there's your answer