1. Lesson 3-9
Weighted Averages

4. Objectives
Solve mixture problems
Solve uniform motion problems

5. Vocabulary
Weighted average – sum of the product of the number of units and the value per unit divided by the sum of the number of units
Mixture problems – problems in which two or more parts are combined into a whole
Uniform motion problems – problems where an object moves at certain speed or rate

6. Mixture Problems
Step 1:

7. Example 1
Pets Jeri likes to feed her cat gourmet cat food that costs $1.75 per pound. However, food at that price is too expensive so she combines it with cheaper cat food that costs $0.50 per pound. How many pounds of cheaper food should Jeri buy to go with 5 pounds of gourmet food, if she wants the price to be $1.00 per pound?
Let w = the number of pounds of cheaper cat food. Make a table.
Units (lb)
Price per Unit
Price
Gourmet cat food
Mixed cat food 5
$1.75
$8.75 w
$0.50
0.5w
5 + w
$1.00
1.00(5 + w)

8. Example 1 cont Original equation Distributive Property
Subtract 0.5w from each side.
Simplify.
Subtract 5.0 from each side.
Simplify.
Divide each side by 0.5.
Simplify.
Answer: Jerry should buy 7.5 pounds of cheaper cat food to be mixed with the 4 pounds of gourmet cat food to equal out to $1.00 per pound of cat food.

9. Amount of Solution (gallons)
Example 2
Auto Maintenance To provide protection against freezing, a car’s radiator should contain a solution of 50% antifreeze. Darryl has 2 gallons of a 35% antifreeze solution. How many gallons of 100% antifreeze should Darryl add to his solution to produce a solution of 50% antifreeze?
Let g = the number of gallons of 100% antifreeze to be added. Make a table.
35% Solution
100% Solution
50% Solution
Amount of Solution (gallons)
Price 2
0.35(2) g
1.0(g)
2 + g
0.50(2 + g)

10. Example 2 cont 0.35(2) 1.0(g) 0.50(2 + g)
Amount of antifreeze in 35% solution
plus
amount of antifreeze in 100% solution
equals
amount of antifreeze in 50% solution.
0.35(2)
1.0(g)
0.50(2 + g)
Original equation
Distributive Property
Subtract 0.50g from each side.
Simplify.
Subtract 0.70 from each side.
Simplify.
Divide each side by 0.50.
Simplify.
Answer: Darryl should add 0.60 gallons of 100% antifreeze to produce a 50% solution.

11. Example 3
Air Travel Mirasol took a non-stop flight from Newark to Austin to visit her grandmother. The 1500-mile trip took three hours and 45 minutes. Because of bad weather, the return trip took four hours and 45 minutes. What was her average speed for the round trip?
To find the average speed for each leg of the trip, rewrite .
Going
Returning

12. Example 3 cont Round Trip Definition of weighted average Simplify.
Answer: The average speed for the round trip was about miles per hour.

13. Example 4 53 miles apart Takes 5 miles to stop Takes 3 miles to stop
Rescue A railroad switching operator has discovered that two trains are heading toward each other on the same track. Currently, the trains are 53 miles apart. One train is traveling at 75 miles per hour and the other train is traveling at 40 miles per hour. The faster train will require 5 miles to stop safely, and the slower train will require 3 miles to stop safely. About how many minutes does the operator have to warn the train engineers to stop their trains?
Draw a diagram.
53 miles apart
Takes 5 miles to stop
Takes 3 miles to stop

14. Example 4 cont
Let m = the number of minutes that the operator has to warn the train engineers to stop their trains safely. Make a table.
Fast train
Other train r
d = rt t 75 m
75m 40 m
40m
Original equation
Simplify.
Divide each side by 115.
Round to the nearest hundredth.
Convert to minutes by multiplying by 60.
Answer: The operator has about 23 minutes to warn the engineers.

15. Summary & Homework Summary: Homework:
The weighted average of a set of data is the sum of the product of each number in the set and its weight divided by the sum of all the weights
The formula d = rt (distance = rate of change (velocity)  time) is used to solve uniform motion problems
Homework:
none