PSLE
Mrs Fong had a bottle of syrup. She used an equal amount of syrup each day. At the end of the fifth day, she had 2200 mℓ of syrup left. At the end of the ninth day, she had two-thirds of the bottle of syrup left. How many litres of syrup were there in the bottle at first?
Fraction of syrup used after two-thirds of the bottle was left
= 1 - ²₃
= ¹₃

¹₃ of the bottle → 9 days
1 bottle → 9 ÷ ¹₃ = 9 x ³₁ =  27 days

It takes 27 days to finish the whole bottle.

Number of days to use 2200 mℓ of syrup
= 27 - 5
= 22

22 days → 2200 mℓ

1 day → 2200 ÷ 22 = 100 mℓ

27 days → 27 x 100 = 2700 mℓ

1000 mℓ = 1 ℓ

2700 mℓ = 2700 ÷ 1000 = 2.7 ℓ

Volume of syrup in the bottle at first = 2.7 ℓ

Answer(s): 2.7 ℓ