Given 30% and 60% to make 40% solution.
The ratio of vol (30%) : vol (60%) solution is given by
(60-40) : (40-30)
=2:1
So he will need
30*(2/(2+1))=20 L of 30% solution, and
30*(1/(2+1))=10 L of 60% solution.
Solving by algebra:
Let x=volume of 30% solution, then
(30/100)x+(60/100)(30-x) = (40/100)*30
Expand and solve for x
0.3x+0.6(30-x) = 0.4*30
18-12=(0.6-0.3)x
x=6/0.3=20 L (30% solution)
(30-x)=30-20=10 L (60% solution)