Instruction 3-4

Round off Decimals | Compare and Order Positive and Negative Fractions, Decimals, Mixed Numbers | Interpret and Use Ratios | Use Proportions to Solve Problems  | Using LCM to Solve Fraction Problems | Calculate Percentages and Solve Problems

If two ratios form a proportion, the cross products are equal.
For example, if

, then 4 x 9 = y x 6.

Also, if their cross products are equal, the two ratios form a proportion.
For example, if

is a proportion, since 6 x 12 = 8 x 9; both are equal to 72.

To find out if the proportions are equal you need to find the cross product.

The ratios are and .	The cross products are 20 x 10 and 40 x 5.
Compare the cross products.	20 x 10 = 200 40 x 5 = 200
The cross products are equal.
The two ratios are equivalent.

To find fractions proportional to the original solve for the variable given.

Write and solve the proportion.
Cross multiply.	88 = 4n
Solve for n by dividing each side by 4.
Answer	n = 22 ;

Not all proportions are whole numbers.

Write and solve the proportion.
Cross multiply.	420 = 8x
Solve for x by dividing each side by 8.
Answer	n = 52.5 ;

Worksheet 1 Proportional Equations

Worksheet 2 Proportional Equations  

Worksheet 3 Solve Each Proportion.  

Worksheet 4 Solve Each Proportion.   

Worksheet 5 Mixed Exercises.

Links for Students, Parents and Teachers

Now let's do Practice Exercise 3-4 (top). 

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