PSLE
Mrs Song had a bottle of sauce. She used an equal amount of sauce each day. At the end of the fourth day, she had 1680 mℓ of sauce left. At the end of the seventh 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 → 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 1680 mℓ of sauce
= 28 - 4
= 24

24 days → 1680 mℓ

1 day → 1680 ÷ 24 = 70 mℓ

28 days → 28 x 70 = 1960 mℓ

1000 mℓ = 1 ℓ

1960 mℓ = 1960 ÷ 1000 = 1.96 ℓ

Volume of sauce in the bottle at first = 1.96 ℓ

Answer(s): 1.96 ℓ