Multiplying and Dividing Fractions

7th Grade MathAugust, 2013

A Quick Review: KEY POINT when multiplying or dividing

fractions:

Change a mixed number into an improper fraction,

Still simplify your answer.

More on Multiplying Fractions:

“of” means multiply!

Example: There are 8 cars in Michael’s toy collection. 1/2 of the

cars are red. How many red cars does Michael have? This problem is asking “What is 1/2 of 8?” A way to answer it is to put a multiplication sign in place of “of.” You

then get 1/2 x 8 or 8 x ½ (remember that multiplication is commutative).

Multiplying Fractions:

Multiplying fractions is easier than adding or subtraction because you don’t need to find common denominators. YAY!!!!!!

Just multiply straight across. Multiply numerators Then, multiply denominators

A Few Examples: Example #1: 2/3 X 4/5

Answer: 8/15

Example #2: 9/2 X 3/7 Answer: 27/14=1 13/14

Example #3: 2 1/6 X 3/2 Answer: 39/12=3 3/12=3 ¼

Example #4: 5 X 2/7 Answer: 10/7=1 3/7

Example #5:5 1/3 • 9 ½ Answer: 50 2/3

Make Life Easier!! Cross Reduce

When multiplying, you can simplify your factors by “cross reducing”.

Examples: 6/35 • 5/24

2/15 • 3/18

1/8 (4/5)

What in the World is a “Reciprocal”?

Reciprocal: When two fractions are multiplied and their

product is 1. AKA “inverting” or “flipping” a number

Examples: The reciprocal of ½ is _______. The reciprocal of 1 ¾ is _______. The reciprocal of 8 is ________.

123

32

Algorithm: Dividing Fractions

Remember these three words:

KEEP, CHANGE, RECIPROCAL

KEEP First fraction CHANGE Division to multiplication RECIPROCAL of last fraction Multiply and Simplify

Some Examples:

Example #1:

Example #2:

Example #3:

Example #4:

65

43

42

38

31

5

32

241

6

Don’t forget to cross reduce if possible

ONLY when multiplying!

“Card and Domino Multiplication/Division” Each partner draws one (1) card and one (1)

domino Card = whole number Domino = fraction

Multiply your two numbers (individually) Divide your two numbers (individually) Compare your answers. If you both have the

same answer, give your team a point for each problem. Highest points (with work shown) wins candy!