Answer
The number of choices of pens available in three colors and four writing tips = $3_{{C}{_{1}}}$ . $4_{{C}{_{1}}}$ = 12
Work Step by Step
The number of choices of pens available in three colors and four writing tips = $3_{{C}{_{1}}}$ . $4_{{C}{_{1}}}$ = $\frac{3!}{(3 - 1)! 1!}\times\frac{4!}{(4 - 1)! 1!}$
= $\frac{3!}{2! 1!}\times\frac{4!}{3! 1!}$
= $\frac{3\times2!}{2! .1!}\times\frac{4\times3!}{3! .1!}$
= $3\times4$ = 12
