What is the concentration, in w/w%, of a solution prepared from 50.0 grams NaCl and 150.0 grams of water? I think it would be 25.0%, but I'm struggling with these new concentration calculations.

percent w/w means grams solute/100 g soln.

Therefore, you have 50 g solute in 150+50=200 g soln so
(50.0/200)*100 = 25.0%(w/w)