Suppose there are x people who like to eat fish and vegetables. If there is a problem, it means that (15-x) people love fish and vegetables at the same time, (18-x) people love fish and vegetables at the same time, and (13-x) people love meat and vegetables at the same time, so 30-x people only love fish. Only 28-x-(15-x)-(13-x) = x, and only 20-x-(18-x)-(13-x) =

And because the total number of people is 40, the above adds up to 40, that is

X+()-(15-x)+(18-x)+(13-x)+(x-3)+x+(x-1) =