Yup, glad to hear that you also think the OA to the first one should be '10%' Stuart.

Mutual_help,

the formula you used is wrong,

when you say,

n(A int B) + n(A int C) + n(B int C), elements of (A int B int C) are included 3 times in this....

hence, you have to subtract - 2 X n(A int B int C) from the answer you are getting to get the right answer.

hence, it will be, 20 - 2(5) = 10%.

Stuart Kovinsky wrote: Note that this method is really just another way to apply the 3 set equation quoted by Ronnie. If you're an equation person, it's a good equation to know. However, like most questions on the GMAT, you can often reason your way to the correct answer even if you don't know the right formula.

Stuart can you please explain the two problems with help of the equation mentioned by Ronnie. I mean this equation
Total = Group1 + Group2 + Group3 - (sum of 2-group overlaps) - 2*(all three) + Neither