<human>:  Ken can do 20 sit-ups without stopping. Nathan can do twice as many, and Bob can do half the number of Ken and Nathan's combined sit-ups. How many more sit-ups can Bob do compared to Ken?
<bot>: How many sit ups can Nathan do? ** Nathan can do 20 x 2 = <<20*2=40>>40 sit ups.
How many sit-ups can Ken and Nathan do together? ** Ken and Nathan's combined number is 20 + 40 = <<20+40=60>>60 sit-ups.
How many sit-ups can Bob do? ** Bob can do 60/2 = <<60/2=30>>30 sit-ups.
How many more sit-ups can Bob do compared to Ken? ** Bob can do 30 - 20 = <<30-20=10>>10 more sit-ups than Ken.
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