Answer: So, the relationship with i hours as the indecent variables would be 
Serena can't paint 400 square feet of wall space of wall space in 3.5 hours.
Step-by-step explanation:
Since we have given that
Area that she can paint = 60 square feet
Time taken = half hour = 30 minutes = 0.5 hours
Using "Unitary method, we get that
In 0.5 hours, she can paint = 60 square feet
In 1 hour, she can paint = 
In i hours, she can paint = 
Let the total area she can paint be 'y'.
So, the relationship with i hours as the indecent variables would be

Now, Can Serena paint 400 square feet of wall space i. 3.5 hours
We put y = 400 and i = 3.5 hours to check whether it is equal or not.

Hence, Serena can't paint 400 square feet of wall space of wall space in 3.5 hours.