Question 813232
Target is 40% mushrooms.  His current ingredients would yield {{{19/(19+12)=0.6129}}} fraction as mushrooms.  We can use the expression and adjust the target to 0.40 and apply a variable to SUBTRACT from the mushroom quantity of 19.


Let's say, Andrew would use {{{19-w}}} ounces of mushrooms.
{{{(19-w)/((19-w)+12)=0.40}}}
Solve for w, and then compute {{{19-w}}}.


You may find another way to solve this problem.  My solution described is not necessarily the only way.