Question

Suppose one variable is changing while the second is constant. Can this be a proportional relationship?

Answer 1

Question

Whenever two quantities are directly proportional to each other, the ratio between the two quantities is a constant. In our case let's take that first variable - $X_1$ and second is $Y_1$. In some moment of time we will have that: $\frac{X_1}{Y_1} = C_1$. And if our first variable changes in time let take that in some moment of time (which is different from our first moment of time) it will be equal to $X_2$ while the second variable still stays the same - $Y_1$. Now we will have: $\frac{X_2}{Y_1} = C_2$ and $X_1 \neq X_2$. So, we easy can see that $C_1 \neq C_2$. And that's why such case can not be a proportional relationship.

Answer: it can't be a proportional relationship.