## Precalculus (6th Edition) Blitzer

The number of different choices of pens is $20$
Here, it is known that the number of ways in which a series of successive things can occur is found by multiplying the number of ways in which each individual thing can occur. There four choices for ink and five for writing tips. Therefore, the number of choices are $5\times 4=20$ Thus, the number of different choices of pens is $20$.