A mechanic does oil changes from a 50 gallon drum of oil. He uses 4 or 5 quarts of oil per car. About how many oil changes can he do from the 40% of oil left in the drum?

If the mechanic does four quarts/one gallon per car he can change the oil of 20 cars. if he does five quarts per car he can change the oil of 16 cars. That is with the 40% of oil left in his 50 gallon drum.