PSLE
Mrs Kong had a bottle of sauce. She used an equal amount of sauce each day. At the end of the eighth day, she had 1980 mℓ of sauce left. At the end of the tenth 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 → 10 days
1 bottle → 10 ÷ ¹₃ = 10 x ³₁ =  30 days

It takes 30 days to finish the whole bottle.

Number of days to use 1980 mℓ of sauce
= 30 - 8
= 22

22 days → 1980 mℓ

1 day → 1980 ÷ 22 = 90 mℓ

30 days → 30 x 90 = 2700 mℓ

1000 mℓ = 1 ℓ

2700 mℓ = 2700 ÷ 1000 = 2.7 ℓ

Volume of sauce in the bottle at first = 2.7 ℓ

Answer(s): 2.7 ℓ