Because 3.4 is the sum of the weight of oil and bottle, oil can be ÷ 2; Bottles can't be ÷2.

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1, original, oil+bottle = 3.4kg

2. Oil, half used, oil+bottle = 1.9 kg.

3. So the oil used is 3.4- 1.9= 1.5 kg.

4. So the crude oil in the bottle is 1.5× 2 = 3kg.

5. So, the weight of the bottle is 3.4-3=0.4 kg.