PSLE
Mrs Ng had a bottle of sauce. She used an equal amount of sauce each day. At the end of the fifth day, she had 1760 mℓ of sauce left. At the end of the ninth day, she had two-thirds of the bottle of sauce left. How many litres of sauce were there in the bottle at first?
Fraction of sauce 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 1760 mℓ of sauce
= 27 - 5
= 22

22 days → 1760 mℓ

1 day → 1760 ÷ 22 = 80 mℓ

27 days → 27 x 80 = 2160 mℓ

1000 mℓ = 1 ℓ

2160 mℓ = 2160 ÷ 1000 = 2.16 ℓ

Volume of sauce in the bottle at first = 2.16 ℓ

Answer(s): 2.16 ℓ