PSLE
Mrs Yeo had a bottle of detergent. She used an equal amount of detergent each day. At the end of the eighth day, she had 840 mℓ of detergent left. At the end of the tenth day, she had half of the bottle of detergent left. How many litres of detergent were there in the bottle at first?
Fraction of detergent used after half of the bottle was left
= 1 - ¹₂
= ¹₂

¹₂ of the bottle → 10 days
1 bottle → 10 ÷ ¹₂ = 10 x ²₁ =  20 days

It takes 20 days to finish the whole bottle.

Number of days to use 840 mℓ of detergent
= 20 - 8
= 12

12 days → 840 mℓ

1 day → 840 ÷ 12 = 70 mℓ

20 days → 20 x 70 = 1400 mℓ

1000 mℓ = 1 ℓ

1400 mℓ = 1400 ÷ 1000 = 1.4 ℓ

Volume of detergent in the bottle at first = 1.4 ℓ

Answer(s): 1.4 ℓ