heres the calculation:
for balloon: n = PV/RT = (1.0atm*3.5L)/(.0821(L*atm/mol*K)298K) = 0.143 moles in 1 balloon
for 150L tank: 1850 psi (68.046x10^(-3) atm / 1psi) = 125.885 atm
n = PV/RT = (125.885atm*15.0L)/(.0821(L*atm/mole*k)298K) = 771.8 moles in tank.
then 771.8 moles / 0.143 moles = 5397.2 ~5.4x10^4