**If the percentage composition is by volume** then the ratio of number of moles of component gases will be ratio of their respective volume.

Hence the ratio of number of moles of #H_2,O_2and He# will be as follows

#n_(H_2):n_(O_2):n_(He)=20:15:65=4:3:13#

So mole fractions of component gases

#X_(H_2)=4/(4+3+13)=1/5=0.2#

#X_(O_2)=3/(4+3+13)=1/20=0.15#

#X_(He)=65/(4+3+13)=13/20=0.65#

And the partial pressures of component gases

#p_(H_2)=X_(H_2)xxP=0.2xx4atm=0.8atm#

#p_(O_2)=X_(O_2)xxP=0.15xx4atm=0.6atm#

#p_(He)=X_(He)xxP=0.65xx4atm=2.6atm#

**When the percentage composition is by weight** then the ratio of number of moles of component gases #H_2,O_2and He# will be as follows

#n_(H_2):n_(O_2):n_(He)#

#=20/("molar mass of " H_2):15/("m. mass of " O_2):65/("m.mass of " He)#

#=20/2:15/32:65/4#

#=4/2:3/32:13/4#

#=64:3:104#

Now we are to proceed similar way as above

And the partial pressures of component gases

#p_(H_2)=X_(H_2)xxP=64/171xx4atm~~1.5atm#

#p_(O_2)=X_(O_2)xxP=3/171xx4atm~~0.07atm#

#p_(He)=X_(He)xxP=104/171xx4atm~~2.43atm#