check the picture below
so, the lateral, sides, area, is just the area of the sides, well, the sides are just those three rectangles
now, the rectangle is the middle is length = 25 as well as the others, but we dunno the width, say width = c
[tex]\bf c=\sqrt{16^2+12^2}\implies c=\sqrt{256+144}\implies c=\sqrt{400}\implies c=20\\\\
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\textit{lateral area}\implies (25\cdot 16)+(25\cdot 12)+(25\cdot 20)
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400+300+500[/tex]
