A 100-mL object has a mass in air of 50 g.  The object is put into a jar from which all the air is pumped out (vacuum).
How much does the object weigh in the vacuum?
(Density of air = 0.00118-g/mL)

The mass of the object will greater in a vacuum, therefore you will need to add the mass of the air to the objects mass in air.

Mass of body
50-g
50-g
Volume of body
100-mL
 .
Volume of fluid displaced
10-mL
 .
Mass of fluid displaced 
(Salt water has a density of 0.00118-g/mL)
+ 0.118-g
- 0.118-g
Mass of body while water (subtract) .
 50.118-g

Please note that due to significant figures, the 0.118-g is not significant and should not be in the answer.

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