Final answer:
Suijin babysat for 5 hours last week.
Step-by-step explanation:
Suijin babysat for 20 hours this week, which is 4 times as long as she babysat last week. To find out how long she babysat last week, we can divide 20 by 4. 20 divided by 4 is 5, so Suijin babysat for 5 hours last week.