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Answer:
- A: less than 2 hours
- B: 2 to 5 hours
- C: more than 5 hours
Step-by-step explanation:
The attached graph shows the various company costs for x number of hours. The graph nearest the x-axis represents the lowest cost.
We can see that cost is lowest using Company A for 2 hours or less, and Company C for 5 hours or more. For times between those, Company B has the lowest charges.
Of course, the equation for charges in each case is the sum of the service fee and the product of hourly charge and number of hours (x).
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I find the graphing calculator to be the most efficient tool for solving these. The alternative is to compare the equations pairwise to see which gives lower rates. With a little practice, you learn that the "break even hours" will be the difference in service fees divided by the difference in hourly cost.
For example A will cost the same as B when the $20 service fee and the $10/hour cost difference are the same: for 2 hours. A and C will cost the same when the $45 service fee and the $15/hour cost difference are the same, after 3 hours. B and C will cost the same when the $25 difference in service fees and the $5/hour cost difference are the same, after 5 hours.
So B is cheaper above 2 hours, and C is cheaper than that above 5 hours. With no service fee, A is cheaper for small numbers of hours (<2).
