PSLE
Mrs Koh had a bottle of oil. She used an equal amount of oil each day. At the end of the fourth day, she had 1920 mℓ of oil left. At the end of the seventh day, she had three-quarters of the bottle of oil left. How many litres of oil were there in the bottle at first?
Fraction of oil used after three-quarters of the bottle was left
= 1 - ³₄
= ¹₄

¹₄ of the bottle → 7 days
1 bottle → 7 ÷ ¹₄ = 7 x ⁴₁ =  28 days

It takes 28 days to finish the whole bottle.

Number of days to use 1920 mℓ of oil
= 28 - 4
= 24

24 days → 1920 mℓ

1 day → 1920 ÷ 24 = 80 mℓ

28 days → 28 x 80 = 2240 mℓ

1000 mℓ = 1 ℓ

2240 mℓ = 2240 ÷ 1000 = 2.24 ℓ

Volume of oil in the bottle at first = 2.24 ℓ

Answer(s): 2.24 ℓ