In barium sulfate, there is only 58.8% barium mass out of the total mass. From this description you can get this equation:
barium mass= barium sulfate mass * 58.8%
In this question, you are asked the mass of barium in 620mg barium sulfate. Then, the answer would look like this
barium mass= barium sulfate mass * 58.8%
barium mass= 620mg* 58.8%
barium mass= 364.56mg
