I cannot understand how the mass of the system has changed. There's no need for you to do my work please just explain it to me! :) thanks so much

Question

A scientist placed copper powder into a crucible and heated it until all of the copper reacted. The resulting product was cooled and its mass was measured. The data collected during the experiment are shown in the table below.

Mass of Crucible (g)
Mass of Copper + Crucible (g)
Mass of Copper (g)
Mass of Copper(II) Oxide + Crucible (g)
35.0
50.2
15.2
54.0

Answer 1

mass CuO + crucible = 54.0
mass Cu + crucible = 50.2
mass oxygen added = 54.0-50.2 = ?
(NOTE:) As I recall the product is Cu2O and not CuO)