#V_1->"volume of hydrogen in cylinder"=2.82L#

#T_1->"Temperature of hydrogen in cylinder"#

#=27^@C=27+273=300K#

#P_1->"Pressure of hydrogen in cylinder"=20atm#

At NTP

#P_2->"Pressure"=1atm#

#T_2->"Temperarture"=273K#

#V_2->"Volume"=?#

By gas law

#(P_2V_2)/T_2=(P_1V_1)/T_1#

#=>V_2=(P_1V_1T_2)/(P_2T_1)#

#=(20xx2.82xx273)/(1xx300)=51.324L#

Excluding the minlmum remaining 2.82L gas in the cylinder at NTP maximum #(51.324-2.82)L=48.504L# gas can be used for filling baloons.

Volume of each spherical baloon of diameter #7cm# is

#=4/3pi(7/2)^3cm^3=4/3*22/7*7^3/8cm^3#

#=0.1796L#

Hence the maximum number of baloons that can be filled with hydrogen from the cylinder is

#="Total volume of gas"/"volume of a baloon"#

#=48.504/0.1796~~270" "#(rounding to nearest whole number)