Answer: use the ideal gas equation, PV = nRT
Step-by-step explanation:
The ideal gas equation permits you to determine one variable, P, V, n or T, when you know the other two variables.
In this case, your are told you know P, V and T, so you can solve for n:
n = PV / (RT)
Remember this:
R is the universal constant of gases, and you must use the adequate units.
T is the temperature in absolute scales, i.e Kelvin.