Vol. of balloon = 4/3 πr3 = 4851 ml
Hydro. is to be filled up at NTP.
Hence
P1=1 atm. P2=20 atm.
T1=273K T2=300K
V1=? V2=2.82 l
Hence V1= 51324 ml
Actual vol. to be transferred = 51324 - 2820 = 48504ml
No. of balloons = 48504/4851
= 10
If I'm right, pls reply.