Let m be the number of months that have passed so far this year, and let S be the total number of cars Casey has sold so far.
Clue 1: If she sells 20 cars next month
• Months passed: The months will increase by 1, so it becomes m + 1.
• Total cars sold: Her total sales will increase by 20, so it becomes S + 20.
• The average: We are told her average monthly sales will be 17.
Since Average = (Total Sales) / Months, we can write:
(S + 20) / (m + 1) = 17
If we multiply both sides by (m + 1), we get:
S + 20 = 17(m + 1)
S + 20 = 17m + 17
S = 17m - 3 --- (Equation 1)
Clue 2: If she sells 20 cars in each of the next two months
• Months passed: The months will increase by 2, so it becomes m + 2.
• Total cars sold: She sells 20 cars twice (20 + 20 = 40), so her total sales become S + 40.
• The average: We are told her average monthly sales will be 17.5.
Using the average formula again:
(S + 40) / (m + 2) = 17.5
If we multiply both sides by (m + 2), we get:
S + 40 = 17.5(m + 2)
S + 40 = 17.5m + 35
S = 17.5m - 5 --- (Equation 2)
Step 3: Find the number of months (m)
Since both equations equal S, we can set them equal to each other:
17m - 3 = 17.5m - 5
Let's move the m's to one side and the numbers to the other:
5 - 3 = 17.5m - 17m
2 = 0.5m
To find m, divide 2 by 0.5 (which is the same as multiplying by 2):
m = 4
Conclusion
The value m = 4 means that 4 months have passed so far this year.
The first 4 months of the year are January, February, March, and April. Since Casey is tallying her sales on the last day of that 4th month, today must be the last day of April.
Correct Answer: (D) April 30