Question 1190311
"10% plant fertilizer"  ?


If this is all by volumes and liquids, then 

v amount of water
0.5-v amount of 10% fertilizer

{{{10(0.5-v)=4*0.5}}}
-
{{{5-10v=2}}}
{{{5-2=10v}}}
{{{10v=3}}}
{{{v=0.3}}}------------three tenths of a gallon of water