Thank you for the opportunity to help you with your question!
Unit rates are defined as rates with 1 in the denominator. If a rate is given and the quantity in the denominator is not one, then we calculate the unit rate by finding an equal ratio such that the denominator is one. In this case, Julie wrote the rate as $108 in 6 weeks as a unit rate. However, the denominator(6) is not one. Therefore, unit rate = 108/6 = $18 per week.
Please let me know if you need any clarification. I'm always happy to answer your questions! :)