omg i figured it out after, it wasnt too hard it really helped to write it down as like a table

valerie is 16

so basically if monique is 24, then all you need to do is find the year where monique is a certain age (16 in this case) and take half that to have valerie's age.

i tried 18, but that didnt work because half that would be valerie at 9. 24-9 isnt 18.

so i tried 10 to check. valerie would be 5 in this case, 24-5 isnt 10. so then i plugged in 16. valerie would be 8 years old, 24-8 is in fact 16 years old.