Question

How much of 90% ethanol must be mixed with 10% ethanol to make 1 pint of a 40% ethanol solution?

Answer 1

Amount of 90% Ethanol=0.375 pint
Amount of 10% Ethanol=0.625 pint

Because:
90x+10y=40
x+y=1

x=1-y
90(1-y)+10y=40
90-90y+10y=40
90-80y=40
80y=50
y=5/8
x=1-5/8=3/8