ADDITION 5.NBT.7
Addition of Decimals: Same Number of Decimal Places
Purpose: To add decimals having the same number of decimal places
Materials: Decimal Squares, "Blank Decimal Squares for Adding
Decimals - Same Number of Places" (attached), and "Place Value
Tables for Addition" (attached)
TEACHER MODELING/STUDENT COMMUNICATION
Activity 1 Addition with Same Number of Decimal Places
Decimal
Squares
Decimal
Squares
Transparencies
(optional)
1. Select two red Decimal Squares whose total shaded amount is less
than one whole square. Then determine the sum of their two decimals
and write an equation for the sum. Transparent Decimal Squares can
be used to illustrate the combining of shaded amounts.
2. Repeat this activity by having students select pairs of green squares
whose total shaded amount is less than one whole square. Choose a
few students to describe their squares and equations and explain their
reasoning. Discuss the similarity between adding whole numbers and
adding decimals. With the squares, whole numbers of shaded parts
are being added.
3. Select two red squares whose total shaded amount is greater
than or equal to one whole square. Determine the sum of the
two decimals and write an equation for the sum. Select a few
students to explain their reasoning and discuss some of their
examples. Most of their sums will be mixed decimals. If the
total shaded amount of two squares is one whole square, the
sum may be written as 1 or 1.0 to indicate 1 whole square and
0 tenths left over.
Repeat this activity for other pairs of green or red Decimal Squares
with total shaded amounts greater than or equal to one whole square.
4. Select any Decimal Square and write the decimal for its shaded
amount. Then write the decimal for the amount of the square that is
not shaded and write an equation for the sum of the two decimals.
Will this sum always be 1? (Yes)
Activity 2 Shading Blank Decimal Squares for Sums
activity
sheet
1. Distribute "Blank Decimal Squares for Adding Decimals - Same
Number of Places". Shade the first and second squares on the sheet
for .5 and .7, shade the third and fourth squares for the sum (total
shaded amount), and write the equation for the sum below the
squares.
+
=
2. Repeat this activity on your sheet for the following pairs of twoplace decimals: 2. .43 plus .76; and 3. .38 plus .64
Activity 3 Place Value Tables and Regrouping
Decimal
Squares
Place
Value
Tables
for
Addition
Find Decimal Squares for .75 and .45 and compute the sum of
the decimals. (1.20) Illustrate the addition with a place value
table and explain the regrouping. (For example, there is a total
of 10 hundredths in the hundredths column and 10 hundredths
equal 1 tenth. So a zero is recorded in the hundredths column
and a 1 is regrouped to the tenths column.) There is a total 12
tenths, so 2 tenths is recorded in the tenths column and 10
tenths are regrouped by writing a 1 in the units column.) Have
the students use their place value tables for computing other
sums where regrouping is needed.
Activity 4 Approximations by Rounding
Blank
Decimal
Squares
for
Adding
Decimals
Same
Number
of Places
1. Look at the hundredths squares on the activity sheet for
.43 and .74, round each decimal to the nearest tenth, and
compute the sum of the rounded decimals. Discuss that
each column of a decimal square represents one tenth.
Since the partially shaded column for .43 is less than half
shaded, .43 rounds to .4, and since the partially shaded
column for .76 is more than half shaded, .76 rounds to .8.
The sum of these two rounded decimals is 1.2.
2. Repeat this activity by looking at the hundredths squares
for .38 and .64, round each decimal to the nearest tenth, and
add the two rounded decimals.
INDEPENDENT PRACTICE AND ASSESSMENT
Worksheets 5.NBT.7 #1 and #2
decimalsquares.com Decimal Squares
Blackjack (At the Beginner Level,
Decimal Squares Playing Cards for tenths
are used to get close to a sum of 2 without
going over. In this round, Blake got a sum
of 1.9 to win over the Robot's sum of 1.7.)
3
Place Value Tables for Addition Name:
Date:
__
3
.