PSLE
Mrs Yang had a bottle of oil. She used an equal amount of oil each day. At the end of the seventh day, she had 1000 mℓ of oil left. At the end of the ninth day, she had two-thirds of the bottle of oil left. How many litres of oil were there in the bottle at first?
Fraction of oil 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 1000 mℓ of oil
= 27 - 7
= 20

20 days → 1000 mℓ

1 day → 1000 ÷ 20 = 50 mℓ

27 days → 27 x 50 = 1350 mℓ

1000 mℓ = 1 ℓ

1350 mℓ = 1350 ÷ 1000 = 1.35 ℓ

Volume of oil in the bottle at first = 1.35 ℓ

Answer(s): 1.35 ℓ