A barrel of oil weighs 21 kg. If you sell 1 / 3 kg, how much is this barrel of oil Wrong, one third sold, 15kg left

Suppose the barrel of oil weighs x kg
21-1/3X=15
1/3X=21-15
X=6÷1/3
X=18
A: this barrel of oil weighs 18 kg

A barrel of oil weighs 20 kg. After 50% of the oil is removed, the remaining barrel weighs 11 kg. How many kg of oil is there in the barrel? How many kilos does the barrel weigh?

(20-11) △ 50% = 9 △ 50% = 18 (kg) 20-18 = 2 (kg) answer: oil is 18 kg, barrel is 2 kg