In a group of 40 people, 10 are healthy and every person the of the remaining 30 has either high blood pressure, a high level of cholesterol or both. if 15 have high blood pressure and 25 have high level of cholesterol,
a) how many people have high blood pressure and a high level of cholesterol?

A or B" means
i) the cases in which A is true and B is not true
ii) the cases in which A is not true and B is true
iii) the cases in which both A an B are true

"A xor B" means
i) the cases in which A is true and B is not true
ii) the cases in which A is not true and B is true

So, "xor" excludes the cases in which both A and B are true, whereas ordinary "or" includes those cases.

 Or to make it easier:


 

It states that A U B = 30  Also, A = 15 and B = 25.

Some people have both conditions. These people are members of the set A int B , or in other words, people with both high blood pressure and high cholesterol.

  A int B = 15 + 25 - 30 = 40 - 30 = 10