From the question, we're told that Mrs. Jackson has 14 black pens which is 35% of the total number of pens that she has.
Let x represent the total number of pens, our equation will then become;
[tex]\frac{35}{100}\ast x=14[/tex]Let's go ahead and isolate x by multiplying both sides of the equation by 100 and dividing by 35 afterwards;
[tex]\begin{gathered} 35x=100\ast14 \\ 35x=1400 \\ x=\frac{1400}{35} \\ \therefore x=40 \end{gathered}[/tex]Therefore, Mrs. Jackson has 40 pens.