Julie wants to express $108 in 6 weeks as a unit rate.
Her errors are:
$108 = $54 (Not true)
6 weeks = 3 weeks (Not true).
Instead,
[tex] \frac{\$108}{6\,weeks} \\ = \frac{\$54 \times 2}{3\,weeks \times 2} \\ = \frac{\$54}{3\,weeks} \\ =18 \, \frac{\$}{week} [/tex]
The correct unit rate is 18 $/week.