Postman Stories for Multiplying Integers

What do you think?

• Should a check be represented by a positive number, or a negative one?

What do you think?

• Should a check be represented by a positive number, or a negative one?

• Should a bill be represented by a positive number, or a negative one?

Remember . . .

• Checks are positive numbers.– A check for $3 may be written as 3.

• Bills are negative numbers.– A bill for $7 may be written as -7.

What do you think?

• If the postman brings you mail, do you have more mail than before, or less?

• Should this be a positive number, or a negative one?

• If the postman takes away mail, do you have more mail than before, or less?

• Should this be a positive number, or a negative one?

Remember . . .

• If the postman brings you mail, it is represented by a positive number.– If he brings you 6 pieces of mail,

write 6.

• If the postman takes away mail, it is represented by a negative number.– If he takes away 10 pieces of

mail, write -10.

Let’s practice!

• Suppose the postman brings you 5 pieces of mail. How would you write this number?

Let’s practice!

• Suppose the postman brings you 5 pieces of mail. How would you write this number?

• Suppose each piece of mail was a check for $10. How would you write this number?

Let’s practice!

• Suppose the postman brings you 5 pieces of mail. How would you write this number?

• Suppose each piece of mail was a check for $10. How would you write this number?

• What multiplication problem represents this situation?

Let’s practice!

• Suppose the postman brings you 5 pieces of mail. How would you write this number?

• Suppose each piece of mail was a check for $10. How would you write this number?

• What multiplication problem represents this situation?

5 x 10 = ?

Let’s practice!

• Suppose the postman brings you 5 pieces of mail. How would you write this number?

• Suppose each piece of mail was a check for $10. How would you write this number?

• What multiplication problem represents this situation?

5 x 10 = 50

Check It!

5 x 10 = 50

• Does the answer +50 make sense? Did you gain $50 when he brought you 5 checks for $10 each?

Let’s practice!

• Suppose the postman brings you 5 pieces of mail. How would you write this number?

• Suppose each piece of mail was a bill for $10. How would you write this number?

Let’s practice!

• Suppose the postman brings you 5 pieces of mail. How would you write this number?

• Suppose each piece of mail was a bill for $10. How would you write this number?

• What multiplication problem is this?

• What’s the answer to that multiplication problem?

Let’s Practice!

• The postman brought you 5 pieces of mail.

• Each piece of mail was a bill for $10.

5 X -10 = -50

Does it make sense?

• The postman brings you 5 pieces of mail.

• Each piece of mail is a bill for $10.

• You lost $50. This is the number -50!

• Equation:

5 X -10 = -50• You lost $50. This is the number -50!

Let’s Practice

• When you paid your bills, the postman took away 5 pieces of mail.

• Each piece of mail was a check for $10.

• How can we express this mathematically?

Let’s Practice

• When you paid your bills, the postman took away 5 pieces of mail.

• Each piece of mail was a check for $10.

• How can we express this mathematically?

(Write an expression!)

Let’s Practice

• When you paid your bills, the postman took away 5 pieces of mail.

• Each piece of mail was a check for $10.

• How can we express this mathematically?

(Write an expression!)

-5 X 10 = ?

Let’s Practice

• When you paid your bills, the postman took away 5 pieces of mail.

• Each piece of mail was a check for $10.

• Solution:

-5 X 10 = -50

Let’s Practice

• When you paid your bills, the postman took away 5 pieces of mail.

• Each piece of mail was a check for $10.

Let’s Practice

• When you paid your bills, the postman took away 5 pieces of mail.

• Each piece of mail was a check for $10.

• Write a math problem for this situation

and solve it.

Let’s Practice

• When you paid your bills, the postman took away 5 pieces of mail.

• Each piece of mail was a check for $10.

-5 x 10 = -50

Does it make sense?

• The postman took away 5 pieces of mail.

• Each piece of mail was a check for $10.

• Equation:

5 X -10 = -50• You have $50 less than before. This is the

answer: -50!

Last Practice

• It’s time to bill your customers for your business!

• The postman takes away 5 pieces of mail.

• Each piece of mail is a bill for $10.

Last Practice

• It’s time to bill your customers for your business!

• The postman takes away 5 pieces of mail.

• Each piece of mail is a bill for $10.

• Write an equation for this.

Last Practice

• The postman takes away 5 pieces of mail.

• Each piece of mail is a bill for $10.

• Equation:

-5 X -10 = 50

Does It Make Sense?

• The postman takes away 5 pieces of mail.

• Each piece of mail is a bill for $10.

• Equation:

-5 X -10 = 50You made $50!

That’s why the answer is +50.

Thanks to the original author of this power point.

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