Lesson 7

One Hundred Percent

Let’s solve more problems about percent increase and percent decrease.  

7.1: Notice and Wonder: Double Number Line

Double number line.

What do you notice? What do you wonder?

7.2: Double Number Lines

For each problem, complete the double number line diagram to show the percentages that correspond to the original amount and to the new amount.

  1. The gas tank in dad’s car holds 12 gallons. The gas tank in mom’s truck holds 50% more than that. How much gas does the truck’s tank hold?

    A double number line, 4 evenly spaced tick marks.  Top line, gas in gallons, tick mark labels 0, blank, blank, blank. Bottom line, beginning at first tick mark labels 0%, 50%, 100%, 150%.
  2. At a movie theater, the size of popcorn bags decreased 20%. If the old bags held 15 cups of popcorn, how much do the new bags hold?

    A double number line for “popcorn in cups” with 7 evenly spaced tick marks.
  3. A school had 1,200 students last year and only 1,080 students this year. What was the percentage decrease in the number of students?

    Double number line, number of people, percent.
  4. One week gas was $1.25 per gallon. The next week gas was $1.50 per gallon. By what percentage did the price increase?

    A double number line for “price of gas in dollars” with 9 evenly spaced tick marks.
  5. After a 25% discount, the price of a T-shirt was $12. What was the price before the discount?

    A double number line for “price of shirts in dollars” with 6 evenly spaced tick marks.
  6. Compared to last year, the population of Boom Town has increased 25%.The population is now 6,600. What was the population last year?

    A double number line for "number of people" with 6 evenly spaced tick marks.

7.3: Representing More Juice

Two students are working on the same problem:

A juice box has 20% more juice in its new packaging. The original packaging held 12 fluid ounces. How much juice does the new packaging hold?

  • Here is how Priya set up her double number line.
    A double number line for “juice in fluid ounces” with 8 evenly spaced tick marks.
  • Here is how Clare set up her double number line.
    A double number line for “juice in fluid ounces” with 8 evenly spaced tick marks.

Do you agree with either of them? Explain or show your reasoning.



Clare's diagram could represent a percent decrease. Describe a situation that could be represented with Clare's diagram.

7.4: Protecting the Green Sea Turtle

Green sea turtles live most of their lives in the ocean, but come ashore to lay their eggs. Some beaches where turtles often come ashore have been made into protected sanctuaries so the eggs will not be disturbed.

Image of a green sea turtle.
  1. One sanctuary had 180 green sea turtles come ashore to lay eggs last year. This year, the number of turtles increased by 10%. How many turtles came ashore to lay eggs in the sanctuary this year?

  2. At another sanctuary, the number of nesting turtles decreased by 10%. This year there were 234 nesting turtles. How many nesting turtles were at this sanctuary last year?

Summary

We can use a double number line diagram to show information about percent increase and percent decrease:

Double number line.


The initial amount of cereal is 500 grams, which is lined up with 100% in the diagram. We can find a 20% increase to 500 by adding 20% of 500:

\(\begin{align}500+(0.2)\boldcdot 500 &= (1.20)\boldcdot 500\\&=600\end{align}\)

In the diagram, we can see that 600 corresponds to 120%.

If the initial amount of 500 grams is decreased by 40%, we can find how much cereal there is by subtracting 40% of the 500 grams:

\(\begin{align}500−(0.4)\boldcdot 500 &= (0.6)\boldcdot 500\\&=300\end{align}\)

So a 40% decrease is the same as 60% of the initial amount. In the diagram, we can see that 300 is lined up with 60%.

To solve percentage problems, we need to be clear about what corresponds to 100%. For example, suppose there are 20 students in a class, and we know this is an increase of 25% from last year. In this case, the number of students in the class last year corresponds to 100%. So the initial amount (100%) is unknown and the final amount (125%) is 20 students.

Double number line.

Looking at the double number line, if 20 students is a 25% increase from the previous year, then there were 16 students in the class last year. 

Glossary Entries

  • percentage decrease

    A percentage decrease tells how much a quantity went down, expressed as a percentage of the starting amount.

    For example, a store had 64 hats in stock on Friday. They had 48 hats left on Saturday. The amount went down by 16.

    This was a 25% decrease, because 16 is 25% of 64.

    a tape diagram
  • percentage increase

    A percentage increase tell how much a quantity went up, expressed as a percentage of the starting amount.

    For example, Elena had $50 in the bank on Monday. She had $56 on Tuesday. The amount went up by $6.

    This was a 12% increase, because 6 is 12% of 50. 

    a tape diagram