4 is the greatest common factor.
The least common multiplier (least common factor) of 8 and 20 is 40.
To solve for this you need to find the prime factors of each number
8 is 2x2x2
20 is 2x2x5
then count how many times each number is used in each set
8 has 3 twos
20 has 2 twos and 1 five
if the same number is used in both numbers throw out the repeat numbers from the smaller set
8 has more twos than 20 does, throw out the 2 twos
multiple all remaining numbers together
2x2x2x5=40
Check your work by dividing each number into your result, both should divide into it evenly
40/8=5
40/20=2
40 is the lowest number that both the 8 and the 20 will go into evenly.
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The Least Common Multiple (LCM) for 8 20 48 is 240.
The least common factor of any set of numbers is 1.
The greatest common factor of the numbers 8 and 20 is 4.
The least common multiple of two numbers is the product of the two numbers divided by their greatest common factor. The greatest common factor of 4 and 8 is 8. (See related question "What is the greatest common factor of 4 and 8?" for this calculation.) So, the least common multiple is 4 x 8 ÷ 4 = 8. Common multiples of 4 and 8 include 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, etc. Although 16 is a common multiple, it is not the least common multiple.
The least common factor is a term often mistakenly given to either the greatest common factor (GCF) or the least common multiple (LCM). This term is not often used because it does not describe a useful relationship between numbers. Since 1 evenly divides all integers, 1 is technically the least common factor for any set of integers.8 and 9 are relatively prime, which means that they share no factors aside from 1.