PSLE
Mrs Ng had a bottle of detergent. She used an equal amount of detergent each day. At the end of the sixth day, she had 420 mℓ of detergent left. At the end of the eighth day, she had one-third of the bottle of detergent left. How many litres of detergent were there in the bottle at first?
Fraction of detergent used after one-third of the bottle was left
= 1 - ¹₃
= ²₃

²₃ of the bottle → 8 days
1 bottle → 8 ÷ ²₃ = 8 x ³₂ =  12 days

It takes 12 days to finish the whole bottle.

Number of days to use 420 mℓ of detergent
= 12 - 6
= 6

6 days → 420 mℓ

1 day → 420 ÷ 6 = 70 mℓ

12 days → 12 x 70 = 840 mℓ

1000 mℓ = 1 ℓ

840 mℓ = 840 ÷ 1000 = 0.84 ℓ

Volume of detergent in the bottle at first = 0.84 ℓ

Answer(s): 0.84 ℓ