Question 704921
x is how much 18% saline
y is how much 20% saline


The system to solve:
{{{(x*0.18+y*0.20+40*0.10)/(100)=0.15}}}
{{{x+y+40=100}}}


Note that you may work the rational equation into
18x+20y+40*10=100*15
18x+20y+400=1500
{{{9x+10y+200=750}}}