PSLE
Mrs Soh had a bottle of detergent. She used an equal amount of detergent each day. At the end of the fourth day, she had 2000 mℓ of detergent left. At the end of the eighth 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 → 8 days
1 bottle → 8 ÷ ¹₃ = 8 x ³₁ =  24 days

It takes 24 days to finish the whole bottle.

Number of days to use 2000 mℓ of detergent
= 24 - 4
= 20

20 days → 2000 mℓ

1 day → 2000 ÷ 20 = 100 mℓ

24 days → 24 x 100 = 2400 mℓ

1000 mℓ = 1 ℓ

2400 mℓ = 2400 ÷ 1000 = 2.4 ℓ

Volume of detergent in the bottle at first = 2.4 ℓ

Answer(s): 2.4 ℓ