PSLE
Mrs Ng had a bottle of syrup. She used an equal amount of syrup each day. At the end of the seventh day, she had 2600 mℓ of syrup left. At the end of the eleventh 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 → 11 days
1 bottle → 11 ÷ ¹₃ = 11 x ³₁ =  33 days

It takes 33 days to finish the whole bottle.

Number of days to use 2600 mℓ of syrup
= 33 - 7
= 26

26 days → 2600 mℓ

1 day → 2600 ÷ 26 = 100 mℓ

33 days → 33 x 100 = 3300 mℓ

1000 mℓ = 1 ℓ

3300 mℓ = 3300 ÷ 1000 = 3.3 ℓ

Volume of syrup in the bottle at first = 3.3 ℓ

Answer(s): 3.3 ℓ