i think the answer is 12.5 but I'm not positive.
This problem does not make sense to me, partly because I don't know what they mean by cross section area in this case.
A trough is made from a rectangular strip of sheet metal, 50 cm wide, by bending up at right angles a strip, x cm wide, along two sides. For what value of x is the cross-section area a maximum?
In this problem, you are not given the other length of the rectangular
sheet of metal, so all that can be assumed is that, the two sides be bent up from both ends of the 50cm width.
Therefore the only cross-section analysable is the vertical cross-section
of one side view.
There is also a plan-view cross-section, but we don't have the 2nd length of the rectangular sheet, and a perpendicular side view with a 3rd cross-section.