Question: For how many days does the event last?
Day 1 = 3 cars sold
Day 2 = 3+3 = 6 cars sold
Day 3 = 6+3 = 9 cars sold
Therefore, total car sold = 3n; where n=number of days
$$Hence,\ no.\ of\ days=\frac{total\ cars\ sold}{3}$$
We need to find the total number of cars sold to answer the question stem.
Statement 1: If the event has lasted 2 more days, the average number of cars sold per day would have increased by 3.
This does not provide any new information and as such, we cannot estimate the total number of cars sold. So, statement 1 is NOT SUFFICIENT.

Statement 2: On exactly 9 days during the event, the number of cars for the day is a multiple of 9.
If the no. of days = 9, then total cars sold = multiple of 9
If the no. of days = 12, then total cars sold = multiple of 9
The information provided is not enough to find the exact number of days that the event lasted.
Hence, statement 2 is NOT SUFFICIENT.

Combining both statements together:
None of the statement gives the total number of cars sold. Hence, the total number of days cannot be estimated.
So, both statements combined together are NOT SUFFICIENT.


Answer = option E