You can
put this solution on YOUR website! .
A group of 25 students were asked whether they use either Facebook or Twitter or both. Fifteen of these students use Facebook and twelve use Twitter.
a.How many use Facebook only.
b.How many use Twitter only.
c.How many use both social networking sites?.
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I don't know what is the source if this problem, but the formulation is incorrect.
It is incorrect because it doesn't state that these 15 plus 12 students cover the entire group/population/set of the 25 students.
Without this info the problem can not be solved properly.
So, let us assume that this info is given.
Then the number of students who use both Facebook and Twitter is
(15 + 12) - 25 = 2. (*)
Knowing this, you can calculate the rest on your own.
For details and the proof of validity the formula (*), see the lesson Counting elements in sub-sets of a given finite set in this site.
