PSLE
Mrs Kok had a bottle of syrup. She used an equal amount of syrup each day. At the end of the seventh day, she had 780 mℓ of syrup left. At the end of the tenth day, she had half of the bottle of syrup left. How many litres of syrup were there in the bottle at first?
Fraction of syrup 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 780 mℓ of syrup
= 20 - 7
= 13

13 days → 780 mℓ

1 day → 780 ÷ 13 = 60 mℓ

20 days → 20 x 60 = 1200 mℓ

1000 mℓ = 1 ℓ

1200 mℓ = 1200 ÷ 1000 = 1.2 ℓ

Volume of syrup in the bottle at first = 1.2 ℓ

Answer(s): 1.2 ℓ