A linear cost model with a flat fee and an hourly rate has two parts: a charge paid once and a charge that repeats each hour. If the question gives the number of hours and asks for the total, substitute those hours into the rule and evaluate it in Desmos. Don't stop at the hourly charge; the total includes the flat fee.
Hints
- Hint 1
The input is the number you put into a rule. Here the input is hours, so replace with to find the cost of a 3-hour rental.
- Hint 2
A flat fee is paid once, while an hourly rate repeats for each hour. Multiply the rate by , then add the fee to get the total cost.
Step-by-step
Substitute the hours into the cost rule
Step 1Put 3 in for the hours
In the given rule, counts hours, so use for the 3-hour rental:
The flat fee is the $5.50 paid once. Only the $1.25 hourly rate gets multiplied by the number of hours.
- Step 2
Evaluate the total in Desmos
Type in Desmos, then type . Here names the given rule, and is its input, the number of hours put in. Desmos shows , the total cost including the flat fee. Renting the bike for hours costs $9.25. Choice D.