Question 988862
The description and question are not adequate.  "Solution" must mean "syrup" or an ingredient material containing chocolate other than the milk.  You have both 25% and 60% chocolate stock liquid of some kind, and using the 60% would be more efficient.


m, how much MILK
c, how much CHOCOLATE SOLUTION-or-SYRUP
Wanted:  140 LITERS of 30% chocolate milk
Instead of m for how much milk, use 140-c, because the dining hall should mix 140-c liters of milk with c liters of the 60% chocolate.


{{{((140-c)*0+60*c)/140=30}}}
{{{highlight_green(60c/(140)=30)}}}

{{{6c/14=3}}}
{{{3c/7=3}}}
{{{c=3(7/3)}}}
{{{highlight(c=7)}}}


Seven liters of the 60% chocolate "solution" or syrup, and 133 liters of milk.