Question 974628
Copper will dilute the Zinc.


{{{(40(8/40))/(40+z)=0.10}}}


{{{(40(1/5))/(z+40)=1/10}}}


{{{8/(z+40)=1/10}}}


{{{z+40=80}}}


{{{z=40}}}



NOTE:  This seemed a little easier converting fully into regular fractions than percents or mixing percents with regular fractions.  Usually the choice is yours when you solve, depending on instructions.