Question 43564
So, in 6 quarts, we have 60% of alcohol. We can say we have 3.6 quarts of alcohol. We must stop adding water when the ratio {{{3.6/q = 40/100}}}, where q is the total number of quarts. 

Solve for q:

{{{3.6/q = 40/100}}}
{{{3.6 = 40q/100}}}
{{{360 = 40q}}}
{{{q = 9}}}

Therefore, we must add 9 - 6 = 3 quarts of water.