The proportion to solve is:
[tex]\frac{28}{4}=\frac{21}{n}[/tex]We can cross multiply and do a little algebra to solve for the unknown variable n.
The steps are shown below:
[tex]\begin{gathered} \frac{28}{4}=\frac{21}{n} \\ 28n=4\times21 \\ 28n=84 \\ n=\frac{84}{28} \\ n=3 \end{gathered}[/tex]The unknown variable, n, is:
n = 3
Answern = 3