PSLE
Mrs Sih had a bottle of sauce. She used an equal amount of sauce each day. At the end of the seventh day, she had 500 mℓ of sauce left. At the end of the ninth day, she had one-quarter of the bottle of sauce left. How many litres of sauce were there in the bottle at first?
Fraction of sauce used after one-quarter of the bottle was left
= 1 - ¹₄
= ³₄

³₄ of the bottle → 9 days
1 bottle → 9 ÷ ³₄ = 9 x ⁴₃ =  12 days

It takes 12 days to finish the whole bottle.

Number of days to use 500 mℓ of sauce
= 12 - 7
= 5

5 days → 500 mℓ

1 day → 500 ÷ 5 = 100 mℓ

12 days → 12 x 100 = 1200 mℓ

1000 mℓ = 1 ℓ

1200 mℓ = 1200 ÷ 1000 = 1.2 ℓ

Volume of sauce in the bottle at first = 1.2 ℓ

Answer(s): 1.2 ℓ