At stp (standard temperature and pressure), the temperature is T=0 C=273 K and the pressure is p=1.00 atm. So we can use the ideal gas law to find the number of moles of helium:
[tex]pV=nRT[/tex]
where p is the pressure (1.00 atm), V the volume (20.0 L), n the number of moles, T the temperature (273 K) and [tex]R=0.082 atm L K^{-1} mol^{-1}[/tex] the gas constant. Using the numbers and re-arranging the formula, we can calculate n:
[tex]n= \frac{pV}{RT}= \frac{(1.00atm)(20.0L)}{(0.082 LatmK^{-1}mol^{-1})(273 K)}=0.89 mol [/tex]
