Explanation:
We can use the given function:
C(x) = 15.00 - 0.15x
to find the credit left on the card after using it for 30 minutes:
C(30) = 15.00 - 0.15(30)
= 15.00 - 4.50
= 10.50
Therefore, the credit left on the card after Amy uses it for 30 minutes of a call is $10.50.