How many 1.50-L balloons can be completely filled with 0.603 moles of helium gas at STP?

Supposing STP to be 1 atm and 0°C:

(0.603 mol) x (22.414 L/mol) / (1.50 L / balloon) = 9 balloons

PV=nRT <<<memorize!!

n is moles, R is gas constant (use correct one!), T is temp in K (C+273.15)

Since you have a before and after with the same n &R simplifies to

PV/T initial = nR = PV/T final

Now sub in your numbers and solve

SI Units R values:

8.31446261815324 J⋅K−1⋅mol−1

8.31446261815324 m3⋅Pa⋅K−1⋅mol−1

8.31446261815324 kg⋅m2·K−1⋅mol−1s−2

8.31446261815324×103 L⋅Pa⋅K−1⋅mol−1

8.31446261815324×10−2 L⋅bar⋅K−1⋅mol−1

0.082057 L atm mol-1K-1

62.36L·torr/mol·K = L·mmHg/mol·K

US Customary Units R values:

0.730240507295273 atm⋅ft3⋅lbmol-1°R-1

10.731557089016 psi⋅ft3⋅lbmol-1°R-1

1.985875279009 BTU⋅lbmol-1°Rv-1