The interest is:
\(\displaystyle{I}=\${297.50}-\${260}=\${37.50}\)

Substitute P=$260 and \(\displaystyle{t}=\frac{{3}}{{52}}\) (3 weeks out of 52 weeks in a year) the solve for r: \(\displaystyle\${37.50}=\${260}{\left({r}\right)}\cdot\frac{{3}}{{52}}\)

Divide both sides by $260: \(\displaystyle\frac{{37.50}}{{260}}={\left(\frac{{3}}{{52}}\right)}{r}\)

Multiply both sides by \(\displaystyle\frac{{52}}{{3}}:\) \(\displaystyle{\left(\frac{{37.50}}{{260}}\right)}\cdot{\left(\frac{{52}}{{3}}\right)}={r}\) 2.5=r

In percentage, r=250%

Substitute P=$260 and \(\displaystyle{t}=\frac{{3}}{{52}}\) (3 weeks out of 52 weeks in a year) the solve for r: \(\displaystyle\${37.50}=\${260}{\left({r}\right)}\cdot\frac{{3}}{{52}}\)

Divide both sides by $260: \(\displaystyle\frac{{37.50}}{{260}}={\left(\frac{{3}}{{52}}\right)}{r}\)

Multiply both sides by \(\displaystyle\frac{{52}}{{3}}:\) \(\displaystyle{\left(\frac{{37.50}}{{260}}\right)}\cdot{\left(\frac{{52}}{{3}}\right)}={r}\) 2.5=r

In percentage, r=250%