PSLE
Mrs Pang had a bottle of syrup. She used an equal amount of syrup each day. At the end of the fourth day, she had 1920 mℓ of syrup left. At the end of the seventh day, she had three-quarters of the bottle of syrup left. How many litres of syrup were there in the bottle at first?
Fraction of syrup 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 syrup
= 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 syrup in the bottle at first = 2.24 ℓ

Answer(s): 2.24 ℓ