Adding and Subtracting fractions

Adding and Subtracting fractions

Let's review how to add and subtract fractions

To find the least common denominator, you have to find the smallest number that both the numerator and the denominator will go into evenly. 

For example:
2 and 3 both go into 6
4 and 5 both go into 20
6 and 9 both go into 18

The next slide has a video about finding common denominators.  If you are already comfortable with your level of mastery of this skill, you can skip the video.  If not, it has a really great method to help you find the least common denominator

Multiple Choice

In order to add or subtract fractions, you must have

• more than 3 numbers

• a common denominator

• greatest common factor

• a pizza

Fill in the Blanks

Poll

I am totally cool with finding common denominators

• Absolutely

• Mostly

• Kinda

• Uh....

• Common What now??

Multiple Choice

what is the least common denominator for 

 $\frac{1}{3}$  +  $\frac{2}{9}$  

• 3

• 9

• 12

• 2,658,091,327

Multiple Choice

What is the answer to the previous math problem?


​

 $\frac{1}{3}\ +\ \frac{2}{9}=$  

• 3/12

• 5/9

• 3/9

Multiple Choice

Sometimes when you add fractions, you get an improper fraction as your answer. When that happens, you should:

• make the improper fraction a mixed number

• do nothing to the improper fraction

• eat some ice cream

Multiple Select

 $\frac{5}{6}\ +\ \frac{8}{9}$  

•  $1\ \frac{13}{15}$  

•  $2\ \frac{13}{15}$  

•  $1\ \frac{13}{18}$  

•  $2\ \frac{13}{18}$  

•  $1\ \frac{13}{15}$  

Poll

What do you think you should do if you are adding mixed numbers?

• add the whole numbers together and then add the fractions

• add the whole number to the fraction

• cross multiply the fractions

• stare at them blankly and hope you get abducted by aliens so you don't have to solve the problem.

Multiple Choice

 $4\ \frac{2}{5}\ +\ 3\ \frac{1}{10}$  If you are supposed to add these fractions, what steps would you have to do?

• 1. Find the common denominators 
  2. Make like fractions
  3. Add the fractions
  4. Add the whole numbers
  5. Simplify if possible

• 
  1. Add the fractions
  4. Ignore the the whole numbers
  5. Simplify if possible

• 1. Find the common denominators 
  2. Make like fractions
  3. Add the fractions to the whole numbers
  4. Simplify if possible
  
  
  

Multiple Choice

 $2\ \frac{1}{4}\ \ +\ \ 5\ \frac{1}{6}\ =$  solve this problem 

•  $7\ \frac{5}{12}$  

•  $7\ \frac{2}{10}$  

•  $7\ \frac{2}{24}$  

Subtracting fractions

Subtracting is just like addition.  You have to find a common denominator.  

Multiple Choice

What would be the common denominator for this subtraction problem?

 $\frac{8}{9}\ -\frac{3}{5}$  

• 9

• 5

• 45

• 30

Did you say 45?  Good.  Because 45 is the smallest number that both 9 and 5 will go into evenly.  

Borrowing from a mixed number

If the fraction of the second number is smaller than the fraction in the first number, you have to borrow from the whole number.

For example

Now I can subtract 3/3 - 2/3

Multiple Choice

what is the difference between

 $5\ \frac{1}{3}\ -\ 2\ \frac{2}{3\ }$  ?

•  $2\ \frac{2}{3}$  

•  $3\ \frac{1}{3}$  

Poll

What if you are subtracting a mixed number from a fraction? For example, I have a 5 pound bag of flour and I use

 $3\ \frac{2}{4}$  pounds to make brownies.    How can I take  the fraction away from the whole number?

• Borrow 1 whole from the 3 and write that 1 whole as a fraction that equals 1 whole

• Run out of the room screaming

Borrowing from a whole number to be able to subtract a fraction.

Multiple Choice

Ms. Kessler bought a 5 pound bag of flour.  She used  $3\ \frac{2}{4}$  pounds of flour to make brownies for her class.  How much flour is left over?  

•  $2\ \frac{1}{2}$  

•  $1\ \frac{1}{4}$  

•  $1\ \frac{1}{2}$  

Multiple Choice

Ms. Kessler's dog, Lucy, needs to go on a diet. She weighs 76 pounds and she needs to lose  $12\ \frac{4}{5}$   pounds in order to reach her ideal weight.  What is Lucy's ideal weight?

•  $64\ \frac{1}{4}$  

•  $88\ \frac{4}{5}$  

•  $63\ \frac{4}{5}$  

•  $63\ \frac{1}{5}$  

Multiple Choice

6 - 2 $\frac{2}{5}$  

• 4  3/5

• 8  2/5

• 3   3/5

• 3  2/5