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Lesson 10Money and Debts

Let's apply what we know about signed numbers to money.

Learning Targets:

  • I understand what positive and negative numbers mean in a situation involving money.

10.1 Concert Tickets

Priya wants to buy three tickets for a concert. She has earned $135 and each ticket costs $50. She borrows the rest of the money she needs from a bank and buys the tickets.

  1. How can you represent the amount of money that Priya has after buying the tickets?
  2. How much more money will Priya need to earn to pay back the money she borrowed from the bank?
  3. How much money will she have after she pays back the money she borrowed from the bank?

10.2 Cafeteria Food Debt

At the beginning of the month Kiran had $24 in his school cafeteria account. Use a variable to represent the unknown quantity in each transaction below and write an equation to represent it. Then, represent each transaction on a number line. What is the unknown quantity in each case?

  1. In the first week he spent $16 on lunches. How much was in his account then?
  2. Then he deposited some more money and his account balance was $28. How much did he deposit?
  3. Then he spent $34 on lunches the next week. How much was in his account then?
  4. Then he deposited enough money to pay off his debt to the cafeteria. How much did he deposit?
  5. Explain why it makes sense to use a negative number to represent Kiran's account balance when he owes money.

Lesson 10 Summary

Banks use positive numbers to represent money that gets put into an account and negative numbers to represent money that gets taken out of an account. When you put money into an account, it is called a deposit. When you take money out of an account, it is called a withdrawal.

People also use negative numbers to represent debt. If you take out more money from your account than you put in, then you owe the bank money, and your account balance will be a negative number to represent that debt. For example, if you have $200 in your bank account, and then you write a check for $300, you will owe the bank $100 and your account balance will be -$100.

starting balance deposits and withdrawals new balance
0 50 0 + 50
50 150 50 + 150
200 -300 200 + (\text-300)
-100

In general, you can find a new account balance by adding the value of the deposit or withdrawal to it. You can also tell quickly how much money is needed to repay a debt using the fact that to get to zero from a negative value you need to add its opposite.

Glossary Terms

deposit

When you put money into an account, it is called a deposit.

For example, a person added $60 to their bank account. Before the deposit, they had $435. After the deposit, they had $495, because 435 + 60 = 495 .

withdrawal

When you take money out of an account, it is called a withdrawal.

For example, a person removed $25 from their bank account. Before the withdrawal, they had $350. After the withdrawal, they had $325, because 350 - 25 = 325 .

Lesson 10 Practice Problems

    1. Clare has $54 in her bank account. A store credits her account with a $10 refund. How much does she now have in the bank?
    2. Mai owes the bank $60. She gets $85 for her birthday and deposits it into her account. How much does she now have in the bank?
    3. Tyler is overdrawn at the bank by $180. His brother has $70 more than him. How much money does Tyler’s brother have?
    4. Andre has $37 in his bank account and writes a check for $87. After the check has been cashed, what will the bank balance show?

  2. The table shows five transactions and the resulting account balance in a bank account, except some numbers are missing. Fill in the missing numbers.

    transaction amount account balance
    transaction 1 200 200
    transaction 2 -147 53
    transaction 3 90
    transaction 4 -229
    transaction 5 0

  3. In each diagram, x represents a different value. For each diagram,

    1. What is something that is definitely true about the value of x ?

    2. What is something that could be true about the value of x ?

    Four diagrams of number lines labeled “A,” “B,” “C,” and “D” are indicated. Each number line has the numbers negative 1, zero, and 1 labeled. On diagram A, x is less than halfway between zero and negative 1. On diagram B, x is slightly to the right of 1, where 1 is closer to x than it is to 0. On diagram C, negative x is to the left of negative 1, where negative 1 is closer to negative x than it is to 0. On diagram D, negative x is more than halfway between zero and 1.

  4. Decide whether each table could represent a proportional relationship. If the relationship could be proportional, what would be the constant of proportionality?

    1. The number of wheels on a group of buses.

      number of buses number of wheels wheels per bus
      5 30
      8 48
      10 60
      15 90

    2. The number of wheels on a train.

      number of train cars number of wheels wheels per train car
      20 184
      30 264
      40 344
      50 424