I need to know if the problem have an answer or if it was badly posed.

Question

Daniel and Anna had visited Toledo and with their camera they had taken 45 pictures. Daniel appears in 15 of them, Anna in 26 and both in solely 7. In how many photos neither of them appear?

I´d really appreciate an answer.

Answer 1

Hint

Let $X$ the set of all photos, $A$ the set of photos where Daniel appear and $B$ the set of photo where Anna appear. Then, $|X|=45$, $|A|=15$, $|B|=26$ and $|A\cap B|=7$. What is $|X\backslash (A\cup B)|$ ?