Answer :
Firstly she was 20 feet below the surface, then she dove down 3 more feet and then she dove up 7 feet, so:
-20 - 3 + 7 =
= -23 + 7 =
= -16 [feet] (that's her depth)
-20 - 3 + 7 =
= -23 + 7 =
= -16 [feet] (that's her depth)