In a group of 30 people, how many drink tea but not coffee?

Hello I also agree with your reasoning. That's why I marked OA as A.

sorry, but for me statement 1 means that all those who drink tea drink also coffe, but it doesn't mean necerssarily that all those who drink coffee drink also tea, maybe the number of persons who drink coffee is greater than the one for both.

Yes I absolutely agree with you. But you are answering the wrong question. The question here is asking "How many people drink tea but not coffee?". From statement 1 we know that all of those people who drink tea also drink coffee. It follows that you CAN NOT find a person in the party who drinks tea but doesn't drink coffee, since the statement 1 would be crushed. If there is someone who drinks tea but not coffee, how it could be possible that all of the people who drink tea also drink coffee. Therefore, the number of people who drink tea but not coffee has to be zero.

What you are talking about is another issue. The number of people who drink coffee but not tea could go both ways. What I mean by that is what you presented 1) the number of total coffee drinkers could very well be more than the number of people who drink both tea and coffee. The number of both tea and coffee drinkers could be 5 and the number of total coffee drinkers could be 25. So, the number of coffee drinkers but not tea drinkers will be 15. But, 2) the number of both coffee and tea drinkers could be 20 and the total number of coffee drinkers could be also 20. Here we would have in a party there are in total 30 people, 20 of them drink both beverages and 10 of them drink neither. So, the number of both tea but not coffee drinkers and the number of coffee but not tea drinkers could be zero. I mean with a given info you cannot determine whether the total number of coffee drinkers is equal to the both coffee and tea drinkers (both yes and no).

But what you can infer from the statement 1 is that the number of tea but not coffee drinkers has to be zero.