Teaching Algebra with Manipulatives

Using Overhead Manipulatives
(Use with Algebra 1, Lesson 3-7)
Percent of Change
Objective Model percent of increase and decrease.
Materials
• dot paper transparency*
• transparency pens* * available in Overhead Manipulative Resources
Demonstration
Modeling Percent of Change
• Ask students what is meant by the expression the
price of an item was increased by 15% and the price
of an item has been decreased by 10%. Explain that
such percents are referred to as percents of increase
and decrease, or percents of change. Tell students
that percents of change can be modeled with dot
paper.
• On the dot paper transparency, draw a 4-by-5
rectangle. Tell students you want to decrease the
area by 25%. Separate the rectangle into 20 equal
parts as shown in Figure A. Ask students what part
of the rectangle each square is. Since there are
20 squares, each square is one-twentieth of the
rectangle, or 5%. Ask them how many squares
represent 25% of the rectangle. 5 squares
• Remove 5 squares, from the original figure to show a
decrease of 25%. See Figure B.
• Refer students to Figure C. Ask them how its area
compares to the original figure, and to explain how
they determined the percent of change. increase of
25%; 5 more squares were added to the original
figure and each square added was 5% of the
original figure
A B C
Algebra 1—Chapter 3
• Draw Figures X, Y, and Z on the dot paper
transparency. Ask what percent of the original each
square represents. 1 8 or 121 2 %
Original
X
Decrease
Y
Increase
Z
© Glencoe/McGraw-Hill 77 Teaching Algebra with Manipulatives