let x be the amount of oil which is sold for $1.27
then (90-x) is the amount of oil which is sold for $1.18.
Now you can set up the equation:
. Expand the bracket and collect like terms :
. Divide both sides of the equation by 0.09 and you'll get:
That means: Take 60 ltrs of the $1.27-oil and 30 ltrs of the $1.18-oil to get the mixture which you can sell for $1.24