the ideal gas law equation is as follows
PV = nRT
where P - pressure
V - volume
n - number of moles
R - universal gas constant
T - temperature
so if the pressure, volume and temperature are already known
we are left with n and R
since R is the universal gas constant that has a known fixed value then R too is known
so we are left with 'n'
once we know temperature volume and pressure
we can find the number of moles of gas present using the ideal gas law equation