PSLE
Hazel had a bottle of laundry detergent. She used an equal amount of laundry detergent each day. At the end of the 12th day, ¹₃ of the bottle was left. At the end of the 16th day, the amount of laundry detergent left was 400 mℓ. What was the amount of laundry detergent in the bottle at first? Express your answer in mℓ.
Fraction of the bottle of laundry detergent left after the 12th day
= 1 - ¹₃
= ²₃

16 days - 12 days = 4 days

²₃ of the bottle → 12 days
¹₃ of the bottle → 12 ÷ 2 = 6 days

6 days → 4 days + 400 mℓ
6 days - 4 days → 400 mℓ
2 days → 400 mℓ

1 day → 400 ÷ 2 = 200 mℓ

18 days → 18 x 200 = 3600 mℓ

Amount of laundry detergent in the bottle at first = 3600 mℓ

Answer(s): 3600 mℓ