P=20/112+20/280+20/520+20/832
P=20/8.14+20/14.20+20/20.26+20/26.32
P=10/3.(1/8-1/14+1/14-1/20+1/20-1/26+1/26-1/32)
P=10/3.(1/8-1/32)
P=10/3.(4/32-1/32)
P=10/3.3/32
P=5/16
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