3.8: Problem Solving Strategies: Use a Formula
Learning Objectives
 Read and understand given problem situations.
 Develop and apply the strategy: use a formula.
 Plan and compare alternative approaches to solving problems.
Introduction
In this chapter, we have been solving problems in which quantities vary directly with one another other. In this section, we will look at few examples of ratios and percents occurring in realworld problems. We will follow the Problem Solving Plan.
Step 1 Understand the problem
Read the problem carefully. Once you have read the problem, list all the components and data that are involved. This is where you will be assigning your variables.
Step 2 Devise a plan – Translate
Come up with a way to solve the problem. Set up an equation or formula.
Step 3 Carry out the plan – Solve
This is where you solve the formula you came up with in Step 2.
Step 4 Look – Check and Interpret
Check to see if you used all your information and that the answer makes sense.
It is important that you first know what you are looking for when solving problems in mathematics. Math problems often require that you extract information and use it in a definite procedure. You must collect the appropriate information and use it (using a strategy or strategies) to solve the problem Many times, you will be writing out an equation which will enable you to find the answer.
Example 1
An architect is designing a room that is going to be twice as long as it is wide. The total square footage of the room is going to be 722 square feet. What are the dimensions in feet of the room?
Step 1 Collect Relevant Information.
Step 2 Make an Equation
Step 3 Solve
Solution
The dimensions of the room are 19 feet by 38 feet.
Step 4 Check Your Answer
Is 38 twice 19?
Is 38 times 19 equal to 722?
The answer checks out.
Example 2
A passenger jet initially climbs at 2000 feet per minute after takeoff from an airport at sea level. At the four minute mark this rate slows to 500 feet per minute. How many minutes pass before the jet is at 20000 feet?
Step 1
The first two pieces on information can be combined. Here is the result.
Height at four minute mark
Step 2 Write an equation.
Since we know that the height at four minutes is 8000 feet, we need to find the time taken to climb the final
We will use
Step 3 Solve.
Solution
The time taken to reach 20000 feet is 28 minutes.
Step 4 check your answer
What is 4 times 2000?
What is 24 times 500?
The total climb = initial climb + secondary climb =
The answer checks out.
Example 3
The time taken for a moving body to travel a given distance is given by
Step 1 We will write out the most important information.
Step 2 We will convert this information into equations.
Time in air
Time in water
Step 3 Solve for
Solution
The delay between the two sound waves arriving is 22.7 seconds.
Step 4 Check that the answer works.
We need to think of a different way to explain the concept.
The actual time that the sound takes in air is 29.41 seconds. In that time, it crosses the following distance.
The actual time that the sound takes in water is 6.67 seconds. In that time, it crosses the following distance of.
Both results are close to the 10000 meters that we know the sound traveled. The slight error comes from rounding our answer.
The answer checks out.
Example 4:
Deandra is looking over her paycheck. Her boss took tax from her earnings at a rate of 15%. A deduction to cover health insurance took onetwelfth of what was left. Deandra always saves onethird of what she gets paid after all the deductions. If Deandra worked 16 hours at $7.50 per hour, how much will she save this week?
Step 1 Collect relevant information.
Deductions:
Step 2 Write an equation.
Deandra’s earnings before deductions
Fraction remaining after tax
Fraction remaining after health
Fraction to be saved
Step 3 Solve
Amount to save =
Solution
Deandra saves $31.17.
Step 4 Check your answer by working backwards.
If Deandra saves $31.17, then her takehome pay was
If Deandra was paid $93.51, then before health deductions health she had
If Deandra had $102.01 after tax, then before tax she had
If Deandra earned $120.01 at $7.50 per hour, then she worked for
This is extremely close to the hours we know she worked (the difference comes from the fact we rounded to the nearest penny).
The answer checks out.
Lesson Summary
The four steps of the Problem Solving Plan are:
 Understand the problem
 Devise a plan – Translate
 Carry out the plan – Solve
 Look – Check and Interpret
Review Questions
Use the information in the problems to create and solve an equation.
 Patricia is building a sandbox for her daughter. It is to be five feet wide and eight feet long. She wants the height of the sand box to be four inches above the height of the sand. She has 30 cubic feet of sand. How high should the sand box be?
 A 500 sheet stack of copy paper is 1.75 inches high. The paper tray on a commercial copy machine holds a two foot high stack of paper. Approximately how many sheets is this?
 It was sale day in Macy’s and everything was 20% less than the regular price. Peter bought a pair of shoes, and using a coupon, got an additional 10% off the discounted price. The price he paid for the shoes was $36. How much did the shoes cost originally?
 Peter is planning to show a video file to the school at graduation, but is worried that the distance that the audience sits from the speakers will cause the sound and the picture to be out of sync. If the audience sits 20 meters from the speakers, what is the delay between the picture and the sound? (The speed of sound in air is 340 meters per second).
 Rosa has saved all year and wishes to spend the money she has on new clothes and a vacation. She will spend 30% more on the vacation than on clothes. If she saved $1000 in total, how much money (to the nearest whole dollar) can she spend on the vacation?
 On a DVD, data is stored between a radius of 2.3 cm and 5.7 cm. Calculate the total area available for data storage in square cm.
 If a Bluray
TM DVD stores 25 gigabytes (GB), what is the storage density, in GB per square cm?
Review Answers
 13 inches
 Approximately 6860 sheets
 $50
 0.06 seconds
 Approximately $565

85.45 cm2 
0.293 GB/cm2
Texas Instruments Resources
In the CK12 Texas Instruments Algebra I FlexBook, there are graphing calculator activities designed to supplement the objectives for some of the lessons in this chapter. See http://www.ck12.org/flexr/chapter/9613.
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