PSLE
Mrs Lau had a bottle of detergent. She used an equal amount of detergent each day. At the end of the eighth day, she had 2500 mℓ of detergent left. At the end of the eleventh day, she had two-thirds of the bottle of detergent left. How many litres of detergent were there in the bottle at first?
Fraction of detergent used after two-thirds of the bottle was left
= 1 - ²₃
= ¹₃

¹₃ of the bottle → 11 days
1 bottle → 11 ÷ ¹₃ = 11 x ³₁ =  33 days

It takes 33 days to finish the whole bottle.

Number of days to use 2500 mℓ of detergent
= 33 - 8
= 25

25 days → 2500 mℓ

1 day → 2500 ÷ 25 = 100 mℓ

33 days → 33 x 100 = 3300 mℓ

1000 mℓ = 1 ℓ

3300 mℓ = 3300 ÷ 1000 = 3.3 ℓ

Volume of detergent in the bottle at first = 3.3 ℓ

Answer(s): 3.3 ℓ