PSLE
Mrs Gan had a bottle of vinegar. She used an equal amount of vinegar each day. At the end of the seventh day, she had 1740 mℓ of vinegar left. At the end of the ninth day, she had three-quarters of the bottle of vinegar left. How many litres of vinegar were there in the bottle at first?
Fraction of vinegar used after three-quarters of the bottle was left
= 1 - ³₄
= ¹₄

¹₄ of the bottle → 9 days
1 bottle → 9 ÷ ¹₄ = 9 x ⁴₁ =  36 days

It takes 36 days to finish the whole bottle.

Number of days to use 1740 mℓ of vinegar
= 36 - 7
= 29

29 days → 1740 mℓ

1 day → 1740 ÷ 29 = 60 mℓ

36 days → 36 x 60 = 2160 mℓ

1000 mℓ = 1 ℓ

2160 mℓ = 2160 ÷ 1000 = 2.16 ℓ

Volume of vinegar in the bottle at first = 2.16 ℓ

Answer(s): 2.16 ℓ