Factor them.
2 x 2 = 4
2 x 5 = 10
Combine the factors, eliminating duplicates
2 x 2 x 5 = 20, the LCD
or
List the multiples.
4, 8, 12, 16, 20, 24
10, 20, 30
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20
8
To find an LCD, you need to have at least two denominators.
To find the lcd of any set of numbers, you must find the lowest number which all of the numbers in the set can divide evenly into. For example, to find the lcd of 7,8, and 9, you must first see if there are any numbers that can divide by 2 of the three numbers in the set. An example of such a number is 56. 56/7 = 8 56/8 = 7 Once you have a number like this, the final step is to multiply that number by the other number in the set that does not divide into it (in this case, 9) 56x9 = 504 So, the lcd of 7, 8, and 9 is 504.
The lowest common denominator of 6 and 9 is three.