Petrolyn motor oil is a combination of natural oil and synthetic oil.
it contains 8 liters of natural oil for every 5 liters of synthetic oil.
natural oil / synthetic oil
8 / 5
let x= liters of synthetic oil and
728-x = liters of natural oil
8/5 = 728-x/x , 8x = 5 (728-x)
8x = 3640-5x
8x+5x = 3640
13x = 3640
x = 280 litres
280 liters of synthetic oil is needed to make 728 liters of petrolyn oil.