Bunuel wrote:
A month with 31 days has the same number of Mondays and Wednesdays. How many of the seven days of the week could be the first day of this month?
A. 2
B. 3
C. 4
D. 5
E. 6
Let’s say the month is March (since it has 31 days) and let’s test each day of the week for 3/1.
If 3/1 is Monday, then 3/3 is Wednesday and 3/29 is Monday and 3/31 is Wednesday. We see that in this case, we have the same number of Mondays and Wednesdays. (Note: Here we are trying to look for the earliest Monday and Wednesday of the month and the latest Monday and Wednesday of the month and we want the following sequence: Monday, Wednesday, Monday, Wednesday.)
If 3/1 is Tuesday, then 3/2 is Wednesday and 3/7 is Monday and 3/28 is Monday and 3/30 is Wednesday. We see that in this case, we have one more Wednesday than Monday.
If 3/1 is Wednesday, then 3/6 is Monday and 3/27 is Monday and 3/29 is Wednesday. We see that in this case, we have one more Wednesday than Monday.
If 3/1 is Thursday, then 3/5 is Monday and 3/7 is Wednesday and 3/26 is Monday and 3/28 is Wednesday. We see that in this case, we have the same number of Mondays and Wednesdays.
If 3/1 is Friday, then 3/4 is Monday and 3/6 is Wednesday and 3/25 is Monday and 3/27 is Wednesday. We see that in this case, we have the same number of Mondays and Wednesdays.
If 3/1 is Saturday, then 3/3 is Monday and 3/5 is Wednesday and 3/26 is Wednesday and 3/31 is Monday. We see that in this case, we have one more Monday than Wednesday.
If 3/1 is Sunday, then 3/2 is Monday and 3/4 is Wednesday and 3/25 is Wednesday and 3/30 is Monday. We see that in this case, we have one more Monday than Wednesday.
So 3/1 (or the first day of the month) can be Monday, Thursday or Friday.
Answer: B