PSLE
Mrs Choy had a bottle of vinegar. She used an equal amount of vinegar each day. At the end of the seventh day, she had 1200 mℓ of vinegar left. At the end of the ninth day, she had two-thirds of the bottle of vinegar left. How many litres of vinegar were there in the bottle at first?
Fraction of vinegar used after two-thirds of the bottle was left
= 1 - ²₃
= ¹₃

¹₃ of the bottle → 9 days
1 bottle → 9 ÷ ¹₃ = 9 x ³₁ =  27 days

It takes 27 days to finish the whole bottle.

Number of days to use 1200 mℓ of vinegar
= 27 - 7
= 20

20 days → 1200 mℓ

1 day → 1200 ÷ 20 = 60 mℓ

27 days → 27 x 60 = 1620 mℓ

1000 mℓ = 1 ℓ

1620 mℓ = 1620 ÷ 1000 = 1.62 ℓ

Volume of vinegar in the bottle at first = 1.62 ℓ

Answer(s): 1.62 ℓ