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Welcome to IRSC’s LIVE Virtual Lesson

on:Adding and Subtracting

Fractions with

Like and Unlike Denominators

Instructor: Lara DiMartino

How to participate in this session:

Raising your hand

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What you will learn today:1. How to add fractions with like (common) 

denominators.

2. How to subtract fractions with like (common) denominators.

3. How to add fractions with unlike denominators.

4. How to subtract fractions with unlike denominators.

5. How to reduce/simplify fractions.

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Why learn these skills?

• Fractions were invented long before decimal 

numbers as a way of showing portions less 

than 1.

• They're used in baking, in building, in sewing, 

in the stock market ­ they're everywhere!  

So, we need to understand them. 

Parts of a Fraction

Numerator

______________      Vinculum

Denominator

Also  written this way:

Numerator/Denominator

Understanding the Parts

Numerator:

• The top number. 

• It is the number of parts you have.• It functions as the dividend when dividing.

Denominator:

• The bottom number.

• It is the total number of parts of the whole. • It functions as the divisor of the numerator when dividing.

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Understanding Fractions

• The whole numbers are the multiples of 1. 

(1, 2, 3, 4, 5, and so on…)

• The fractions are its parts:  its halves, thirds, 

fourths, fifths, and so on.

( 1/2,  1/3 , 1/4,  1/5, …)

1 written in fraction form

This is 1 whole pizza.            

There are 8 individual slices (parts).

These parts goes in the numerator spot.

There are 8 slices that make up the (whole) pizza.

This whole goes in the denominator spot.

1 is equal to 8/8.

A fraction is a part of 1.

A fraction has a value that is less than 1.

Example: This pizza shows 3 parts (numerator) 

left of the whole 4 pieces (denominator). 

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What are Like Denominators?

Denominators that are the same.

They have the same value.

Example: 

• 3/4 and 1/4 both have 4 as denominators.

• So, they have “like” denominators also known as “common denominators”.

What are Unlike Denominators?

Denominators that are different.

Example:

2/6 and 2/12 have different denominators.

They have “unlike denominators”.

Adding Fractions with Like Denominators

Step 1: Add the numerators.

Step 2: Keep the original common denominator for the answer.

2/8

+   1/8_

3/8

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Subtracting Fractions with Like Denominators

Step 1: Subtract the numerators.

Step 2: Keep the original common denominator for the answer.

8/10

­ 1/10

7/10

Instructional Video

More on Like Denominators…

Adding Fractions with Unlike Denominators

Step 1: Find a common denominator that both original denominators can divide into evenly.

Step 2: Put in the new common denominator.

Step 3: Calculate the new numerators.

Step 4: Add the new numerators.

Step 5: Keep the new common denominator for the   answer.

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Step 1: Find a Common Denominator

Example:  3/4 + 1/8 = 

1. First, list the multiples of each denominator. 

4: { 4, 8, 12, 16…}

8: { 8, 16, 24…}

2. Then look for the smallest number, that they have in common, that appears in each list.  (This number (8) has to be evenly divisible by both original denominators (4 and 8).

Our new common denominator is : 8!!!!

Step 2: Put in the new common denominator.

3/4 ­>  ? /8

+ 1/8 ­>  ?/ 8

Step 3: Calculate the new numerator(s).

Example:  

3/4 /8    8/4 = 2  THEN 2 X 3 = 6        6/8

+ 1/8    /8   (Keep the same)                 + 1/8

1. Divide the original denominator (4) into the new one (8).

2. Take that quotient (2) and multiply it by the old numerator (3) . That product (6) is the new numerator.

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Step 4: Add the New Numerators

6/8  

+ 1/8

7/?

Step 5: Use the new common denominator.

6/8  

+ 1/8

7/8

Subtracting Fractions withUnlike Denominators

Use the same steps as when 

adding, but of course, 

subtract instead.

Example:

4/7  

­ 2/5

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Step 1: Find a common denominator.

7: { 7, 14, 21, 28, 35…}

5: { 5, 10, 15, 20, 25, 30, 35…}

Both original denominators (7 and 5) must be able to divide evenly into the new common denominator.

Step: 2 Put in the new common denominator.

4/7 ­>    ? /35

­ 2/5 ­>    ? /35

Step 3: Calculate the new numerators.

4/7   ? /35   35/7= 5  then 5 X 4 =  20/35

­2/5   ? /35  35/5= 7 then 7 X 2 =   ­ 14/35

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Step 4: Subtract the new numerators.

20/35

­ 14/35

6/?

Step 5: Use the new common denominator.

20/35

­ 14/35

6/35

Is Your Answer in Simplest Terms?

If not, you need to simplify (reduce).

Example: 

5/12

+ 1/12

6/12  

This can be reduced!

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Reducing continued-What number can divide evenly into

both 6 and 12? 

Determine the (GCF)greatest common factor.

Factors of 6:   {1, 2, 3, 6}

Factors of 12: { 1, 2, 3, 4, 6, 12} 

6 can divide into both!  

6_         6 =  1

12          6 =  2     

½ is the answer in simplest terms!

Instructional Video

More on how to reduce!

Any Questions?

Type your questions in the chat 

window please for whiteboard 

practice.

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Print your participant window.

• Why? To email to your instructor as proof of attendance. To get 1 hour of credit towards your 10 hours this week.

• How? Place your cursor and left click your mouse on the participant window. On your keyboard, hold down the SHIFT and PRINT SCREEN keys. Then open a Word document and paste (Ctrl + V). Last, attach your word document to an email and send it to your instructor.

Final Comments• This session has been recorded for you to play

back and view at any time.

• If you have any questions regarding this topic at a later time, don’t hesitate to contact your instructor.

• Don’t forget to use the Smarthinking tutor feature within your class site. A tutor is available to you 24 hours a day.

Thank you for coming!

I hope you will take advantage of our future LIVE virtual lessons and will attend some of those sessions as well.

Have a great day!

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