PSLE
Mrs Oh had a bottle of detergent. She used an equal amount of detergent each day. At the end of the sixth day, she had 2040 mℓ of detergent left. At the end of the tenth day, she had three-quarters of the bottle of detergent left. How many litres of detergent were there in the bottle at first?
Fraction of detergent used after three-quarters of the bottle was left
= 1 - ³₄
= ¹₄

¹₄ of the bottle → 10 days
1 bottle → 10 ÷ ¹₄ = 10 x ⁴₁ =  40 days

It takes 40 days to finish the whole bottle.

Number of days to use 2040 mℓ of detergent
= 40 - 6
= 34

34 days → 2040 mℓ

1 day → 2040 ÷ 34 = 60 mℓ

40 days → 40 x 60 = 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