Question 14264
You should start with 2 variables.  "D" for dumped and "P" for poured (for the two liquids)
To solve, you need to use either substitution or elimination.
Set up your first equation. 
{{{D+P=50}}}
Because you are pouring two solutions to make one of 50 mL, you can say that equation is true. 
The second equation is a little tricky. 
You take the percent and change it to decimals to make the second equation. 
{{{.1D+.4P=.16(50)}}}
How did I get that?
Let's say the 10% bromine solution is D and the 40% solution is P
You want to mix 10% and 40% to get 16% and that 16% solution is 50 mL. 
That's how I got the {{{.16(50)}}} part
Then you put them together
{{{D+P=50}}}
{{{.1D+.4P=.16(50)}}}
You can either use substitution or elimination. For this problem, I personally think substitution is much easier. 
Isolate one variable.
{{{D=50-P}}}
{{{.1(50-P)+.4P=8}}}
{{{5-.1P+.4P=8}}}
Move isolate the P
{{{.3P=3}}}
{{{P=10}}}
Plug it back in the original equation.
{{{D+10=50}}}
{{{D=40}}}
So your answer should be that you should use 10 mL of the 40% solution and 40 mL of the 10% solution
Hope this helps!