After you write your equation, what should you do first to get the term with the variable by itself: add, subtract, multiply, or divide?

Once you have an equation like $0.15x = 21$, what operation will isolate $x$? Be careful working with the decimal $0.15$.

Should the number of texts be more than 100 or less than 100? Use the per-text cost and the remaining amount after the monthly fee to estimate.

In a new expression line, type (46 - 25) / 0.15 and press Enter. The value that Desmos outputs is the number of text messages sent.

Let $x$ be the number of text messages sent in that month.

The total bill is made of:

• A fixed monthly fee of $25
• Plus $0.15 for each text message

So the cost equation is:

$25 + 0.15x = 46$

where $46 is the total bill in dollars.

To solve for $x$, first subtract the fixed fee $25 from both sides of the equation:

$25 + 0.15x = 46$

$0.15x = 46 - 25$

$0.15x = 21$

Now the equation directly relates the per-text charge to the remaining $21.

Now divide both sides of the equation by $0.15$ to solve for $x$:

$x = \dfrac{21}{0.15}$

Notice $0.15 = \dfrac{15}{100}$, so:

So the customer sent 140 text messages that month.