mass of the gas = 10 g
The law of conservation of mass :
In a closed / isolated system, the total mass of the substance before the reaction will be equal to the total mass of the reaction product.
Reaction
[tex]\tt X+Z\rightarrow gas+solution[/tex]
mass before reaction=mass after reaction
[tex]\tt 25+60=gas+75\\\\gas=25+60-75=10~g[/tex]