Lynn purchased a bottle of detergent. She used ⁴₇ of the detergent in March. She then used the remaining detergent in April and May in the ratio 2 : 5. Given that the amount of the detergent used in May was 0.78 ℓ less than that in March, how many millilitres of detergent did the bottle contain at first?

March	April	May	Total
4x7	3x7		7x7
	2x3	5x3
28 u	6 u	15 u	49 u

Total volume of April and May is combined repeated identity. Make the total volume of April and May the same. LCM of 3 and 7 is 21.

Volume of less detergent syrup in the bottle used in May than March
= 28 u - 15 u
= 13 u

13 u = 0.78 

1 u = 0.78 ÷ 13 = 0.06

Amount of detergent in the bottle at first
= 49 u
= 49 x 0.06
= 2.94 ℓ

1 ℓ = 1000 mℓ

Amount of detergent in the bottle at first
= 2.94 x 1000
= 2940 mℓ

Answer(s): 2940 mℓ