by jnelson0612 Fri Nov 26, 2010 12:15 pm
A double set matrix is not appropriate for this problem because we cannot account for each possibility once and only once in each grouping. In other words, for each row and each column I must be able to contain all members of the set and have that number add up to the total of the set. For example, I could have girls and boys, and each child is either going to a party or not going to a party. We put the "girls" and "boys" across the top, for example, and the "party" and "not party" on the side. This example is appropriate because every member of the set is counted once in each grouping. A child is either a girl or a boy, and that child is either going or not going to the party.
In this instance, my first group is "rain" and "no rain". That group is okay because all members of the set either did or did not get rain. However, the other grouping is "Monday" and "Tuesday". This one does not work because some counties may have gotten rain on both days. Thus, each individual member is not counted only once in this grouping.
For this type of problem, it is better to use this formula:
Total = Group1 + Group2 + neither - both
Assume there are 100 counties. 70 counties received rain on Monday, 65 received rain on Tuesday, and 25 did not receive rain on either day. Plug into the formula:
100=70 + 65 + 25 - both
100=160 - both
both = 60
Thus, answer choice D is correct.
Thank you,
Jamie Nelson
ManhattanGMAT Instructor