Question 1195288
Wine is never as much as  "80% alcohol" by volume nor by 'weight'.


The arithmetic you are looking for may go like this:

{{{(0.8*40-0.8v)/40=0.7}}}

{{{-0.8v=0.7*40-0.8*40}}}


{{{v=(0.8*40-0.7*40)/0.8}}}


{{{highlight(v=5)}}}