We assume a mass of 100*g.....and work out the molar quantities on that basis........
"Moles of water"=(25*g)/(18.01*g*mol^-1)=1.39*mol
"Moles of ethanol"=(25*g)/(46.07*g*mol^-1)=0.543*mol
"Moles of acetic acid"=(50*g)/(60.05*g*mol^-1)=0.833*mol
And thus...........
chi_(H_2O)=(1.39*mol)/((1.39+0.543+0.833)*mol)=0.503
chi_("EtOH")=(0.543*mol)/((1.39+0.543+0.833)*mol)=0.196
chi_("HOAc")=(1.39*mol)/((1.39+0.543+0.833)*mol)=0.301
And we note that chi_(H_2O)+chi_"EtOH"+chi_"HOAc"=1.00, as is required for a Sigman_i where n_i are the individual molar quantities.