you are combining 50ml of a 20%sol,150ml of a 15% sol.,and 200ml of a 35% sol.what is the final % concentration of the mixed sol. how many total grams are is the final sol?

Grams total= .2*50 + .15*.15 + 200*.35

This is making a big assumption: that the density of the solutions are very close to 1g/ml. For 35percent by mass solutions, this assumption is silly.

final concentration= totalgrams/(50+150+ 200) in g/ml