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#