C2M Lesson 45 Dividing by a Decimal.notebook
January 09, 2015
Bell Assignment
Solve:
1. 7200 ÷ 9
2. $320 ÷ 4
3. 15x = 600
4. 7y = $840
Challenge:
Copy the problem and fill in the missing digits:
_ 6
x _ _
_ _
+ _ _0
2 _ 6
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C2M Lesson 45 Dividing by a Decimal.notebook
January 09, 2015
Lesson 45: Dividing by a Decimal Number
Dan has $2.00 and wants to buy red pens for his friends. If red pens cost $0.25 each, how many can Dan buy?
This is a problem we may be able to solve quickly by mental calculation. Dan can buy 4 red pens for $1.00, so he can buy 8 red pens for $2.00. But how do we get an answer for "8 red pens" from what seems to be a division problem?
In this lesson we will consider how to get the "8." In a later lesson we will consider how to get the "red pens." Notice that dividing $2.00 by $0.25 is dividing by a decimal number ($0.25).
$2.00 $0.25
If we wish to divide by a whole number instead of by a decimal number, we can convert to an equivalent division problem using cents rather than dollars.
200 cents 25 cents 2
C2M Lesson 45 Dividing by a Decimal.notebook
January 09, 2015
Changing from dollars to cents shifts the decimal point two placed to the right. The units (cents over cents) cancel, and 200 divided by 25 is 8.
200 cents = 8
25 cents
Recall that we can form equivalent division problems by multiplying (or dividing) the dividend and divisor by the same number. We use this method to change "division by a decimal" problems to "division by a whole number" problems.
If we want to divide 1.36 by 4, we have
The quotient of 13.6 divided by 4 is the same as the quotient of 1.36 divided by 0.4. This means both of these division problems have the same answer.
To divide by a decimal number, we move the decimal point in the divisor to the right to make the divisor a whole number. then we move the decimal point in the dividend the same number of places to the right.
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C2M Lesson 45 Dividing by a Decimal.notebook
January 09, 2015
Example 1:
Divide: 3.35 ÷ 0.05
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C2M Lesson 45 Dividing by a Decimal.notebook
January 09, 2015
Example 2:
Divide: 0.144 ÷ 0.8
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C2M Lesson 45 Dividing by a Decimal.notebook
January 09, 2015
Example 3:
Solve: 0.07x = 5.6
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C2M Lesson 45 Dividing by a Decimal.notebook
January 09, 2015
Example:
Divide: 21 ÷ 0.5
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C2M Lesson 45 Dividing by a Decimal.notebook
January 09, 2015
Example:
Divide: 1.54 ÷ 0.8
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C2M Lesson 45 Dividing by a Decimal.notebook
January 09, 2015
Example 6: How many $0.35 erasers can be purchased with $7.00?
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C2M Lesson 45 Dividing by a Decimal.notebook
January 09, 2015
Homework: Lesson 45: a­g & 8­28
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