Explanation:
According to gas equation  PâVâ/Tâ = PâVâ/Tâ
Where Pâ is the initial pressure , Vâ is the initial volume and Tâ is the initial temperature
Pâ is the final pressure , Vâ is the final volume and Tâ is the final temperature
Thus Pâ = 15 atmosphere  , Vâ = 12 liters and Tâ = 273 + 20 = 293 K
Pâ = ? , Vâ = 8.5 liters , Tâ = 273 + 35 = 308 K
From gas equation Pâ = PâVâTâ/TâVâ = 15 x 12 x 308/ 293 x 8.5
= 22.3 atmosphere .
If the volume is doubled
The gas equation will be PâVâ = PâVâ ; Â because temperature is constant
Here Vâ = 2 Vâ  therefore  pressure Pâ = Pâ/2 = 22.3/2 = 11.1 atmosphere