moles of benzene and toluene both are same means equal amount of both are mixed

hence we know that mole fraction is number of moles divided by total number of moles of solution 

mole fraction (Xb) for benzene Nb/Nb+Nt

molefraction (Xt )for toluene Nt/Nb+Nt

Nb----no. of moles of benzene

Nt----no. of moles of toluene

given:

Nb=Nt     =>Xb=Xt-------(1)

Xt+Xb=1  --------(2)

from 1&2

2Xt=1

Xt=1/2=0.5

Xt=Xb=0.5