Check the picture below.
as you can see, the graph of the volume function comes from below goes up up up, reaches a U-turn then goes down down, U-turns again then back up to infinity.
the maximum is reached at the close up you see in the picture on the right-side.
Why we don't use a higher value from the graph since it's going to infinity?
well, "x" is constrained by the lengths of the box, specifically by the length of the smaller side, namely 5 - 2x, so whatever "x" is, it can't never zero out the smaller side, and that'd happen when x = 2.5, how so? well 5 - 2(2.5) = 0, so "x" whatever value is may be, must be less than 2.5, but more than 0, and within those constraints the maximum you see in the picture is obtained.
Kevin painted
of the wall.
Solution:
Fraction of the wall painted by Elena = 
Fraction of the wall painted by Matthew = 
Fraction of the wall painted by Kevin = ?
Full wall can be taken as 1.
<u>To find the wall painted by Kevin:</u>
Rest of the wall = Full wall – painted by Elena – Painted by Matthew


To make the denominator same, multiply the numerator and denominator of the first term by 9.



Rest of the wall = 
Hence the kevin painted
of the wall.
Exactly, Thank you again for these quotes!
-p.s you are loved <3
EDIT: THANKS FOR THE POINTS >:3
1/20x52/1=52/20 which equals to your answer that is 2 3/5