The key is to carefully identify the group that the question wants us to exclude.
There are
x total people who won an Olympic medal for water competitions.
One-third of the winners earned a medal for swimming.
So the number of swimming medalists is:
x/3
One-fourth of the swimming medalists also earned a medal for diving.
So the number of people who earned medals for
both swimming and diving is:
1/4 × x/3 = x/12
The question asks how many people won an Olympic medal for water competitions but did
not both receive a medal for swimming and diving.
So we subtract the “both swimming and diving” group from the total:
x - x/12 = 12x/12 - x/12 = 11x/12
Therefore, the correct answer is
A. 11x/12.
Takeaway: The phrase “did not both” means we should exclude only the people who received medals in both swimming and diving, not everyone who received either a swimming medal or a diving medal.
Bunuel
This year, x people won an Olympic medal for water competitions. One-third of the winners earned a medal for swimming and one-fourth of those who earned a medal for swimming also earned a medal for diving. How many people won an Olympic medal for water competitions but did not both receive a medal for swimming and a medal for diving?
(A) 11x/12
(B) 7x/12
(C) 5x/12
(D) 6x/7
(E) x/7