$0.30x=y should be the answer because there is a 30cent fee for every day and x represents how many days it's kept therefore you would add 30 cents for every day it's kept so it's valid to say $0.30x=y becaue the 30 cents times the total of days it's kept = y (the toatl)
Your sister would be 66. When you are 10, your sister would be 6 (10-4=6) 60 years later, you would be 70, and your sister would still be 4 years younger.
