As Jenna weighed 250 pounds in August, this will represent 100%
[tex]\begin{gathered} 250lb\rightarrow100percent \\ 280lb\rightarrow xpercent \end{gathered}[/tex]This relation leads to the next equation
[tex]\begin{gathered} x_{\text{percent}}=\frac{280lb\cdot100_{\text{percent}}}{250lb} \\ \Rightarrow x_{\text{percent}}=\frac{28000_{\text{percent}}}{250} \\ \Rightarrow x_{\text{percent}}=112_{\text{percent}} \end{gathered}[/tex]Therefore, the increment in weight is given by
[tex]x_{\text{percent}}-100=12_{\text{percent}}[/tex]So, the percent increase is 12%
A way of making sure our answer is correct is by doing the next operation
[tex]250(1+0.12)=250(1.12)=280[/tex]