ANSWER
r = 10
EXPLANATION
To solve for r first we have to put r in the numerator. To do that, we have to multiply both sides of the proportion by r:
[tex]\begin{gathered} \frac{28}{35}\cdot r=\frac{8}{r}\cdot r \\ \frac{28}{35}r=8 \end{gathered}[/tex]Now, we have to multiply both sides by 35:
[tex]\begin{gathered} \frac{28}{35}r\cdot35=8\cdot35 \\ 28r=280 \end{gathered}[/tex]And finally divide both sides by 28:
[tex]\begin{gathered} \frac{28r}{28}=\frac{280}{28} \\ r=10 \end{gathered}[/tex]