In a linear cost model, the number multiplying usage is a rate, while the number added on its own is the starting charge. To interpret that added number, set usage to zero and read the charge left. A fixed fee stays when usage is zero. Don't mistake a charge in dollars for a rate in dollars per minute.
Hints
- Hint 1
Zero usage means because counts minutes. In a linear model, that input gives the starting charge. What does the formula give when you use it?
- Hint 2
The per-minute charge is multiplied by the number of minutes, so it disappears at zero minutes. The constant term is the number not multiplied by . What charge remains?
Step-by-step
Find the charge at zero minutes
Step 1Choose the input for no usage
Since counts minutes used, before any usage means . The charge at input zero is called the starting value.
- Step 2
Read the charge that remains
Type to enter the given formula as a function, then type to evaluate it at . Desmos shows . Since is measured in dollars, the is the fixed monthly fee before any usage charges. The multiplies minutes, so it's the per-minute rate instead. Choice C.