Shiyun purchased a bottle of oil. She used ⁷₉ of the oil in January. She then used the remaining oil in February and March in the ratio 1 : 2. Given that the amount of the oil used in March was 0.68 ℓ less than that in January, how many millilitres of oil did the bottle contain at first?

January	February	March	Total
7x3	2x3		9x3
	1x2	2x2
21 u	2 u	4 u	27 u

Total volume of February and March is combined repeated identity. Make the total volume of February and March the same. LCM of 2 and 3 is 6.

Volume of less oil syrup in the bottle used in March than January
= 21 u - 4 u
= 17 u

17 u = 0.68 

1 u = 0.68 ÷ 17 = 0.04

Amount of oil in the bottle at first
= 27 u
= 27 x 0.04
= 1.08 ℓ

1 ℓ = 1000 mℓ

Amount of oil in the bottle at first
= 1.08 x 1000
= 1080 mℓ

Answer(s): 1080 mℓ