A barrel of oil weighs 40 kg. The first time I used 2 / 5, the second time I used the remaining 3 / 5, and the second time I used how much of this barrel of oil?

A barrel of oil weighs 40 kg. The first time I used 2 / 5, the second time I used the remaining 3 / 5, and the second time I used how much of this barrel of oil?

（1－1/3）×3/5＝2/5

A barrel of oil weighs 50 kg. I ate 2 / 5 of it for the first time and 3 / 10 of it for the second time. How many parts of this barrel of oil did I eat in two times?
How many kilos did you eat?

50 * (3 / 5) = 30 * (7 / 10) = 21
So (50-21) / 50 = 29 / 50

A barrel of oil used up 3 / 10 of it for the first time, and the remaining 2 / 7 for the second time. How many parts are left?

Residual = (1-3 / 10) × (1-2 / 7) = 1 / 2