PSLE
Mrs Chew had a bottle of vinegar. She used an equal amount of vinegar each day. At the end of the sixth day, she had 840 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 840 mℓ of vinegar
= 20 - 6
= 14

14 days → 840 mℓ

1 day → 840 ÷ 14 = 60 mℓ

20 days → 20 x 60 = 1200 mℓ

1000 mℓ = 1 ℓ

1200 mℓ = 1200 ÷ 1000 = 1.2 ℓ

Volume of vinegar in the bottle at first = 1.2 ℓ

Answer(s): 1.2 ℓ