### Solution:

Concept- A ratio is a fraction that is expressed as a:b, but a proportion says that two ratios are equal. In this case, a and b can be any two integers. An equation known as proportion shows that the two ratios presented are equal to one another.As given in the question we have the ratio of two liquid and we need to calculate the amount of liter so that it became the ratio of As we have the ratio of two liquids that is So we will take the quantity of liquid as So now we have given the 9 liters of mixture are drawn off now the leftover liquid will be Liquid A left And B will be So the ratio of leftover liquid will be Cross multiplying this will give,

So, we got the value of then the initial value of A is Hence, the correct option is 3) 21