Cool But Can prove wrong using simple maths!
Rep+ for sharing this Mate!
This is how...
Top "triangle" has:
one 8x3 triangle, area 12
two "P" shaped pieces, total area 15
one 5x2 triangle, area 5
for a total of 32.
Bottom picture has those same four pieces, plus the one square
unit hole, for a total of 33.
Thus neither of them can actually fill the 5x13 triangle exactly.
The other explanations do a good job of showing that the reason they
don't fit is that the "hypotenuse" is not a straight line. The two
small triangles have slopes of 3/8 and 2/5, which aren't equal to the
overall apparent slope of that side, 5/13.