Question 1002036
You should figure by intuition that the answer quantity must be 10 mL.


You are adding water, which contains essentially no "acid".
{{{(10*10)/(w+10)=5}}}, a good way to setup the equation, but you appear to have picked decimal fractions instead of percents.  Fine.  That will be this way, instead:


{{{(0.10*10)/(w+10)=0.05}}}


{{{(0.10*10)=0.05(w+10)}}}


{{{1.0=0.05w+0.5}}}


{{{1-0.5=0.05w}}}


{{{0.5=0.05w}}}


{{{(0.5)/(0.05)=w}}}


{{{w=50/5}}}-----just to show a whole number denominator


{{{highlight(w=10)}}}


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Look for an arithmetic mistake in your steps.