The temperature, pressure and volume of two gases X and Y are T, P and V, respectively. When the gases are mixed then the volume and temperature of mixture remains V and T, respectively. The pressure and mass of the mixture will be : 

2P and 2M

From Dalton's law,

Pressure of mixture = P₁ + P₂ = P + P = 2P

Similarly, mass also will become double i.e. 2M