Question 333345
35% NaCl solution  let this be x L
50% NaCl solution. this will be 10-x

mix to get 40% 10 L

..
0.35x+0.5(10-x)=0.4*10
0.35x+5-0.5x=4
-0.15x=-1
x=-1/0.15
= 6.66 L  35% solution
Balance 3.34 L will be 50% solution

CHECK
6.66*35+3.34*50 = 40*10