At least 2/3 of the 40 members of a committee must vote in favor of a resolution for it to pass. What is the greatest number of members who could vote against the resolution and still have it pass?
Dividing 40/3 gives us 13.33333, thus 2/3 = 26.6666.. So in order for it to pass, at least 26.6 people need to vote on it, but of course we need an integer so at least 27 people need to vote on it (so, in practice, a bigger portion than 2/3).. That leaves us with 13 people that can vote against and still have it pass.
The non-integer throws us off and in some cases makes us round down. I think that's what they primarily want to test us on in this case, if we understand that we should round UP for required votes instead of round down.