PSLE
Mrs Chan had a bottle of vinegar. She used an equal amount of vinegar each day. At the end of the fourth day, she had 1400 mℓ of vinegar left. At the end of the eighth 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 → 8 days
1 bottle → 8 ÷ ¹₃ = 8 x ³₁ =  24 days

It takes 24 days to finish the whole bottle.

Number of days to use 1400 mℓ of vinegar
= 24 - 4
= 20

20 days → 1400 mℓ

1 day → 1400 ÷ 20 = 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