其它
A container had $27$ ℓ of longan drink. The drink is made up of three $2$-ℓ\textbf{~}bottles of longan syrup and some water. What is the volume of water used to make the drink? 
Overseas Competition->Knowledge Point->Calculation Modules->Unit Conversion->Converting between Units of Volume and Capacity
$$2 \times 3 = 6$$  $$27 - 6 = 21$$ 