PSLE
Mrs Song had a bottle of sauce. She used an equal amount of sauce each day. At the end of the eighth day, she had 4000 mℓ of sauce left. At the end of the twelfth day, she had three-quarters of the bottle of sauce left. How many litres of sauce were there in the bottle at first?
Fraction of sauce used after three-quarters of the bottle was left
= 1 - ³₄
= ¹₄

¹₄ of the bottle → 12 days
1 bottle → 12 ÷ ¹₄ = 12 x ⁴₁ =  48 days

It takes 48 days to finish the whole bottle.

Number of days to use 4000 mℓ of sauce
= 48 - 8
= 40

40 days → 4000 mℓ

1 day → 4000 ÷ 40 = 100 mℓ

48 days → 48 x 100 = 4800 mℓ

1000 mℓ = 1 ℓ

4800 mℓ = 4800 ÷ 1000 = 4.8 ℓ

Volume of sauce in the bottle at first = 4.8 ℓ

Answer(s): 4.8 ℓ