what's the submerge weight of 30 lbs (in air) plexiglass with 45 lbs (in air) aluminum frame in water?

Question

what kind of formula should be used ?

Answer 1

30 pounds of plex has a volume of 0.403826808 cu.ft.
45 pounds of al has a volume of 0.1779830740447759 cu.ft.
or 0.581809882 cu.ft total
water is 62.4 #/cu.ft so this much water weighs
36.3049366
So the weight of the unit in water is about 38.7 pounds (75-the weight of the water displaced:36.3