Gwen purchased a bottle of vinegar. She used ³₇ of the vinegar in February. She then used the remaining vinegar in March and April in the ratio 4 : 5. Given that the amount of the vinegar used in April was 0.21 ℓ less than that in February, how many millilitres of vinegar did the bottle contain at first?

February	March	April	Total
3x9	4x9		7x9
	4x4	5x4
27 u	16 u	20 u	63 u

Total volume of March and April is combined repeated identity. Make the total volume of March and April the same. LCM of 4 and 9 is 36.

Volume of less vinegar syrup in the bottle used in April than February
= 27 u - 20 u
= 7 u

7 u = 0.21 

1 u = 0.21 ÷ 7 = 0.03

Amount of vinegar in the bottle at first
= 63 u
= 63 x 0.03
= 1.89 ℓ

1 ℓ = 1000 mℓ

Amount of vinegar in the bottle at first
= 1.89 x 1000
= 1890 mℓ

Answer(s): 1890 mℓ