PSLE
Mrs Yong had a bottle of syrup. She used an equal amount of syrup each day. At the end of the fourth day, she had 500 mℓ of syrup left. At the end of the sixth day, she had one-third of the bottle of syrup left. How many litres of syrup were there in the bottle at first?
Fraction of syrup used after one-third of the bottle was left
= 1 - ¹₃
= ²₃

²₃ of the bottle → 6 days
1 bottle → 6 ÷ ²₃ = 6 x ³₂ =  9 days

It takes 9 days to finish the whole bottle.

Number of days to use 500 mℓ of syrup
= 9 - 4
= 5

5 days → 500 mℓ

1 day → 500 ÷ 5 = 100 mℓ

9 days → 9 x 100 = 900 mℓ

1000 mℓ = 1 ℓ

900 mℓ = 900 ÷ 1000 = 0.9 ℓ

Volume of syrup in the bottle at first = 0.9 ℓ

Answer(s): 0.9 ℓ