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Course: 5th grade>Unit 4

Lesson 5: Adding and subtracting fractions with unlike denominators word problems

Add and subtract fractions: FAQ

How do we estimate fractions to add or subtract them?

One way to estimate fractions is to round them to the nearest half or whole number. For example, start fraction, 2, divided by, 3, end fraction rounds to start fraction, 1, divided by, 2, end fraction, and start fraction, 7, divided by, 8, end fraction rounds to 1.
Try it yourself with this exercise:

How do you add and subtraction fractions visually?

To add or subtract fractions, they need to have the same denominator. We can use fraction bars to visually compare the fractions so we can add and subtract.
A tape diagram is shown with two tapes of equal length. The tape on the top is divided into two equal pieces. Each piece is further divided into 3 smaller pieces for a total of 6 smaller pieces. The first three smaller pieces are shaded purple. The bottom tape is divided into six equal pieces. The first part is shaded green. The next two parts are shaded pink. The shaded parts on both tapes are equal in size. Above the first tape, a curved bracket across the first three shaded parts is labeled three sixths. Below the bottom tape, a curved bracket across the first shaded part is labeled one-sixth. A curved bracket across the next two shaded parts is labeled three-sixths minus one-sixth.
Try it yourself with this exercise:

What's the difference between adding and subtracting fractions with like denominators and adding and subtracting fractions with unlike denominators?

When the denominators are the same, we can add or subtract the numerators directly. When the denominators are different, we need to find a common denominator first.
Try it yourself with these exercises:

How do we find common denominators?

There are a few different ways to find common denominators. One way is to use the least common multiple (LCM) of the two denominators. Another way is to just multiply the two denominators together.
Try it yourself with these exercises:

What are mixed numbers and how do we add and subtract them?

Mixed numbers are numbers that contain both a whole number and a fraction. For example, 2, start fraction, 1, divided by, 2, end fraction is a mixed number. To add and subtract them, we have to find a common denominator. Then, we rewrite the mixed numbers as improper fractions with that denominator. Lastly, we add or subtract the two improper fractions together and simplify the result if needed.
Try it yourself with these exercises:

Want to join the conversation?

• Thank you so much for this! It helped a lot! But my only question is does this require LCD?
• least common denominator when both denominators are the same and are the first common denominator
• What is LCD?
• lcd means lowest common denominator
• I don't get how to subtract mixed numbers but I know how to find the least common denominator. I am stuck!
• Step 1. Convert them into improper fractions
Step 2. Find the lcm
Step 3. Subtract
• this is really helpful