Question 206292
let x="ml of added 7% solution"


(3%)(100) + (7%)(x) = (5%)(100 + x)


multiplying by 100 to clear percentages ___ 300 + 7x = 500 + 5x


2x = 200 ___ x = 100



this makes sense ...
___ you would expect that mixing equal volumes of different concentrations would give the average of the concentrations