PSLE
Mrs Ma had a bottle of sauce. She used an equal amount of sauce each day. At the end of the fifth day, she had 960 mℓ of sauce left. At the end of the seventh day, she had two-thirds of the bottle of sauce left. How many litres of sauce were there in the bottle at first?
Fraction of sauce used after two-thirds of the bottle was left
= 1 - ²₃
= ¹₃

¹₃ of the bottle → 7 days
1 bottle → 7 ÷ ¹₃ = 7 x ³₁ =  21 days

It takes 21 days to finish the whole bottle.

Number of days to use 960 mℓ of sauce
= 21 - 5
= 16

16 days → 960 mℓ

1 day → 960 ÷ 16 = 60 mℓ

21 days → 21 x 60 = 1260 mℓ

1000 mℓ = 1 ℓ

1260 mℓ = 1260 ÷ 1000 = 1.26 ℓ

Volume of sauce in the bottle at first = 1.26 ℓ

Answer(s): 1.26 ℓ