PSLE
Mrs Leow had a bottle of vinegar. She used an equal amount of vinegar each day. At the end of the fourth day, she had 400 mℓ of vinegar left. At the end of the eighth day, she had one-third of the bottle of vinegar left. How many litres of vinegar were there in the bottle at first?
Fraction of vinegar used after one-third of the bottle was left
= 1 - ¹₃
= ²₃

²₃ of the bottle → 8 days
1 bottle → 8 ÷ ²₃ = 8 x ³₂ =  12 days

It takes 12 days to finish the whole bottle.

Number of days to use 400 mℓ of vinegar
= 12 - 4
= 8

8 days → 400 mℓ

1 day → 400 ÷ 8 = 50 mℓ

12 days → 12 x 50 = 600 mℓ

1000 mℓ = 1 ℓ

600 mℓ = 600 ÷ 1000 = 0.6 ℓ

Volume of vinegar in the bottle at first = 0.6 ℓ

Answer(s): 0.6 ℓ