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The given function for the credit left on the prepaid card is:

\[
C(x) = 30.25 - 0.09x
\]

Where:
- $C(x)$ is the remaining credit after $x$ minutes of calls.
- $x$ is the number of minutes used.
- Bob uses the card for 30 minutes, so $x = 30$.

Now, substitute $x = 30$ into the function to find the remaining credit:

\[
C(30) = 30.25 - 0.09 \times 30
\]

Calculate:

\[
C(30) = 30.25 - 2.70 = 27.55
\]

Thus, the remaining credit on the card after 30 minutes of calls is **$27.55**.

Would you like more details on this solution or have any other questions?

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Here are 5 related questions you might be interested in:
1. How much credit is left if Bob uses the card for 60 minutes of calls?
2. How many minutes of calls can Bob make before his balance reaches zero?
3. What is the credit left after 10 minutes of calls?
4. How much would Bob have to pay for 45 minutes of calls at the same rate?
5. If the call rate increases to 12 cents per minute, how does this affect the credit balance?

**Tip:** For linear functions like this, the slope represents the rate of change (in this case, cost per minute), and the y-intercept is the starting value (initial credit).

How much credit is left on the card after Bob uses it for 30 minutes of calls?

Learn how to calculate the remaining credit on a prepaid phone card after 30 minutes of calls using the linear function C(x) = 30.25 - 0.09x. This problem covers key algebra concepts and the use of linear functions. Suitable for students in Grades 7-9.

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