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The Wonder Number Lessons

Lesson No. 15: Least Common Denominator

Finding the least common denominator (LCD) is an essential first step in adding fractions with unlike denominators.  Finding the least common denominator for two fractions on the Wonder Number Board is simple because all of the factoring information for each number is included in each individual block.

In the following exmaples the chip represents the numerator and the number you place it on is the denominator.

The first two fractions that we will find the least common denominator for are 1/2 and 1/3.  To begin place one blue chip on 2 to represent 1/2, and one red chip on 3 to represent 1/3.  The first number on the board that has both 2 and 3 as a factor will be the least common denominator.  Searching with a viewer you will find that the first number that meets the criteria is 6, which is a product of the factors 2 and 3 (see fig 78).  6 is the least common denominator of 2 and 3.

The next two fractions that we will find the least common denominator for are 1/4 and 1/5.  To begin place one yellow chip on 4 to represent 1/4, and one green chip on 5 to represent 1/5.  The first number on the board that has both 4 and 5 as a factor will be the least common denominator.  Searching with a viewer you will find that the first number that meets the criteria is 20, which is a multiple of the factors 4 and 5 (see fig 79).  20 is the least common denominator of 4 and 5.

The next two fractions that we will find the least common denominator for are 1/4 and 1/6.  To begin place one yellow chip on 4 to represent 1/4, and one red chip on 6 to represent 1/6.  The first number on the board that has both 4 and 6 as a factor will be the least common denominator.  Searching with a viewer you will find the first number that meets the criteria is 12, which is a multiple of both 4 and 6 (see fig 80).  12 is the lowest common denominator of 4 and 6.