Final answer:
To find the efficient number of times per year the house should be cleaned when the cleaner charges a fee of $180 and $120, we equate the total demand function to the fee charged by the cleaner. The efficient number of times is 40 when the fee is $180 and 60 when the fee is $120.
Step-by-step explanation:
To find the efficient number of times per year the house should be cleaned, we need to determine the point at which the total demand for cleaning equals the cost of hiring a cleaner. We can do this by equating the total demand function (sum of individual demands) to the fee charged by the cleaner. Let's solve for when F = $180 and F = $120:
For F = $180:
PA + PB + PC = F
(60 - Q) + (100 - Q) + (140 - Q) = 180
300 - 3Q = 180
3Q = 120
Q = 40
For F = $120:
PA + PB + PC = F
(60 - Q) + (100 - Q) + (140 - Q) = 120
300 - 3Q = 120
3Q = 180
Q = 60
So, when F = $180, the efficient number of times per year the house should be cleaned is 40. When F = $120, the efficient number is 60.
Illustrating this on a diagram would involve plotting the demand curves and the fee (supply) curve, and finding the point where they intersect. However, since we don't have access to graphing capabilities here, please refer to your textbook or class materials for a visual representation.