14. There are 5 different books: three on mathematics and two on physics. In how many ways can two books be selected: one on mathematics and one on physics?
14. Let's denote the mathematics books as $M_{1}, M_{2}, M_{3}$, and the physics books as $\Phi_{1}$, and $\Phi_{2}$. We obtain the following 6 ways: $M_{1} \Phi_{1}, M_{2} \Phi_{1}, M_{3} \Phi_{1}, M_{1} \Phi_{2}, M_{2} \Phi_{2}, M_{3} \Phi_{2}$. We can also reason as follows: the selection of a mathematics book can be done in three ways, and for each of these ways, there are 2 ways to choose a physics book. In total, we will have $3 \cdot 2=6$ ways.