PSLE
Mrs Pang had a bottle of wine. She used an equal amount of wine each day. At the end of the seventh day, she had 1800 mℓ of wine left. At the end of the ninth day, she had two-thirds of the bottle of wine left. How many litres of wine were there in the bottle at first?
Fraction of wine 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 1800 mℓ of wine
= 27 - 7
= 20

20 days → 1800 mℓ

1 day → 1800 ÷ 20 = 90 mℓ

27 days → 27 x 90 = 2430 mℓ

1000 mℓ = 1 ℓ

2430 mℓ = 2430 ÷ 1000 = 2.43 ℓ

Volume of wine in the bottle at first = 2.43 ℓ

Answer(s): 2.43 ℓ