L15_A
         Pigeonhole Principle(more examples)         
         Example_increasing/decreasing subsequence with >
         = 4 numbers

L15_B
         Example_increasing/decreasing subsequence with >
         = 4 numbers(proof)

L15_C
         Example_wingardium Leviosa

L15_D
         Example_wingardium Leviosa(conti.)
         Example_handshake
         Example_33 rocks in 64 squares