PSLE
Mrs Toh had a bottle of vinegar. She used an equal amount of vinegar each day. At the end of the eighth day, she had 1120 mℓ of vinegar left. At the end of the twelfth day, she had half of the bottle of vinegar left. How many litres of vinegar were there in the bottle at first?
Fraction of vinegar used after half of the bottle was left
= 1 - ¹₂
= ¹₂

¹₂ of the bottle → 12 days
1 bottle → 12 ÷ ¹₂ = 12 x ²₁ =  24 days

It takes 24 days to finish the whole bottle.

Number of days to use 1120 mℓ of vinegar
= 24 - 8
= 16

16 days → 1120 mℓ

1 day → 1120 ÷ 16 = 70 mℓ

24 days → 24 x 70 = 1680 mℓ

1000 mℓ = 1 ℓ

1680 mℓ = 1680 ÷ 1000 = 1.68 ℓ

Volume of vinegar in the bottle at first = 1.68 ℓ

Answer(s): 1.68 ℓ