Nathan can do 20 x 2 = <<20*2=40>>40 sit ups.
Ken and Nathan's combined number is 20 + 40 = <<20+40=60>>60 sit-ups.
Bob can do 60/2 = <<60/2=30>>30 sit-ups.
Bob can do 30 - 20 = <<30-20=10>>10 more sit-ups than Ken.
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