PSLE
Mrs Fong had a bottle of vinegar. She used an equal amount of vinegar each day. At the end of the eighth day, she had 3600 mℓ of vinegar left. At the end of the twelfth 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 → 12 days
1 bottle → 12 ÷ ¹₄ = 12 x ⁴₁ =  48 days

It takes 48 days to finish the whole bottle.

Number of days to use 3600 mℓ of vinegar
= 48 - 8
= 40

40 days → 3600 mℓ

1 day → 3600 ÷ 40 = 90 mℓ

48 days → 48 x 90 = 4320 mℓ

1000 mℓ = 1 ℓ

4320 mℓ = 4320 ÷ 1000 = 4.32 ℓ

Volume of vinegar in the bottle at first = 4.32 ℓ

Answer(s): 4.32 ℓ