Answer:9 hr in a week
Step-by-step explanation:
Given
katya makes $20 per hour in tutoring and $8.50 per hour working in bookstore
Let us assume katya works for x hours in tutoring and y hr in bookstore
therefore the money made by katya is
total money=20x+8.5y
(b)Tution for one semester is $5736 (16 weeks)
Katya works 21 hours each week in book store
therefore in 16 weeks katya earns =16\times 21\times 8.5=$ 2856
Thus remaining amount can be earned 5736-2856=2880
therefore katya needs to work 
144 hours in 16 weeks
for 1 week katya needs to work 9 hr.