PSLE
Mrs Yap had a bottle of oil. She used an equal amount of oil each day. At the end of the seventh day, she had 2970 mℓ of oil left. At the end of the tenth 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 → 10 days
1 bottle → 10 ÷ ¹₄ = 10 x ⁴₁ =  40 days

It takes 40 days to finish the whole bottle.

Number of days to use 2970 mℓ of oil
= 40 - 7
= 33

33 days → 2970 mℓ

1 day → 2970 ÷ 33 = 90 mℓ

40 days → 40 x 90 = 3600 mℓ

1000 mℓ = 1 ℓ

3600 mℓ = 3600 ÷ 1000 = 3.6 ℓ

Volume of oil in the bottle at first = 3.6 ℓ

Answer(s): 3.6 ℓ