The wording is a bit ambiguous, to me.
Normally, one would want the mole fraction of the solutes, which would be
fraction=molesKBr/(molesKBr+molesNaCl)
However, the wording is not as specific as this. As it is worded, one has to inclue the moles of water, so
fraction=MolesKBr/(moleswater+molesNaCl+molesKBr)
How do you get moles of water?
moleswater=densitywater * volumewater/molmass water