Let in the mixture
"No. of moles of " ""^10B=x " mol"
"No. of moles of " ""^11B=y " mol"
:. "Mole fraction of "^10B=x/(x+y)
So
x xx 10.012+ yxx11.093=(x+y)xx10.811
=> yxx11.093-yxx10.811=x xx10.811-x xx 10.012
=>0.282y=0.799x
=>y/x=799/282
=>y/x+1=799/282+1
=>(x+y)/x=(282+799)/282=1081/282
=>x/(x+y)=282/1081~~0.26
:. "Mole fraction of "^10B=x/(x+y)=282/1081~~0.26