I need help solving this math problem

Question

Tony purchased a prepaid phone for $50.00. Calls cost 20 cents a minute using this card. The credit, C (in dollars), left on the card after it is used for x minutes of calls is given by the following function. 

C(x)= 50.00-0.20x

How much credit is left on the card after tony uses it for 50 minutes of calls?

Answer 1

50-50/5 = 50-10 = 40

Answer 2

For this case, you would want to plug 50 in for x because x represents the number of minutes used. The equation becomes:

C(50)= 50 - .20(50)

Then solve, completing multiplication first:

C(50)= 50 - 10

C(50) = 40

There are 40 credits remaining.