PSLE
Mrs Tan had a bottle of vinegar. She used an equal amount of vinegar each day. At the end of the eighth day, she had 600 mℓ of vinegar left. At the end of the tenth 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 → 10 days
1 bottle → 10 ÷ ¹₂ = 10 x ²₁ =  20 days

It takes 20 days to finish the whole bottle.

Number of days to use 600 mℓ of vinegar
= 20 - 8
= 12

12 days → 600 mℓ

1 day → 600 ÷ 12 = 50 mℓ

20 days → 20 x 50 = 1000 mℓ

1000 mℓ = 1 ℓ

1000 mℓ = 1000 ÷ 1000 = 1 ℓ

Volume of vinegar in the bottle at first = 1 ℓ

Answer(s): 1 ℓ