PSLE
Mrs Choy had a bottle of vinegar. She used an equal amount of vinegar each day. At the end of the fourth day, she had 640 mℓ of vinegar left. At the end of the sixth 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 → 6 days
1 bottle → 6 ÷ ¹₂ = 6 x ²₁ =  12 days

It takes 12 days to finish the whole bottle.

Number of days to use 640 mℓ of vinegar
= 12 - 4
= 8

8 days → 640 mℓ

1 day → 640 ÷ 8 = 80 mℓ

12 days → 12 x 80 = 960 mℓ

1000 mℓ = 1 ℓ

960 mℓ = 960 ÷ 1000 = 0.96 ℓ

Volume of vinegar in the bottle at first = 0.96 ℓ

Answer(s): 0.96 ℓ