In general, the dependent variable will be by itself on the left side of the equation. Its value depends upon that of the independent variable embedded in some expression on the right side.
It means, central processing unit
She has 10 liters left. Find 4/9 of 18 by multiplying 4/9 and 18. 4/9 of 18 is 8, so she served 8 of the 18, leaving 10 liters.
