Question

Tracy received a $21.00 gift card for phone apps. She has used $9.00 of the value and wants to buy one more app from the list shown to use up the balance. Complete the bar diagram and use the equation $21.00 = x + $9.00 to determine which app she should buy.

The solution is , so Tracy should buy the app.



Convince Me!

What do you notice about the expression on the left side of an equation compared to the expression on the right side when a value is substituted for the variable? How do you know which value is a solution?

Answer 1

The solution is $12, so Tracy should buy the $12 app.

Work: 21 = x + 9 → x = 21 − 9 = 12. The bar diagram shows a whole bar of 21 split into a 9 piece and a 12 piece.

Convince Me!
- When you substitute a value for the variable, you evaluate both sides of the equation. If they produce the same number, the equation is true for that value.
- A value is a solution if, after substitution, the left and right expressions are equal. For example, substituting x = 12 gives 21 on the left and 12 + 9 = 21 on the right, so x = 12 is a solution. If the two sides are not equal after substitution, the value is not a solution.