<p>hello</p>
<p>"club a has 15 students, club b has 12, only 11 students belong to only one club. how many students belong to 2 clubs"</p>
<p>thanks</p>
<p>Think of the number of “club memberships.” There are a+b = 15+12 = 27 total “memberships” in clubs. Now only 11 of those memberships are held by one student, so 27-11 = 16 are held by more than one student. Since we are only talking about TWO clubs, then each of the remaining students is in two clubs: 16 /2 = 8. </p>
<p>Does that help?</p>
<p>One problem with the question is that it does not actually say the ones who belong to two clubs means they belong to a and b but assuming that is the case then you get an easy equation: let X equal number that belong to both clubs:</p>
<p>27= 11 + 2(X) </p>
<p>In other words there are 27 membership seats total when you add up the total membership for both clubs, but X number fill two of those seats each and 11 fill only one of those seats each.</p>
<p>We need to subtract the number of students who are in both clubs from both clubs’ membership records. The number of students in a single club is 11. So, if we let b = # students in both clubs…</p>
<p>15 - b + 12 - b = 11
27 - 2b = 11
b = 8</p>