Core Building - No Gym - No time

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Looking for some kind of core strength exercise I can do given the following "conditions"

I have roughly 10 mins to exercise each set usually on a my breaks while my coffee is brewing (currently I fit in between 100-200 push-ups each day during these times by doing 50 each time)

I have no equipment but I do have a good bit of floor space.

I can't do crunches because I have a bad back and really bad neck and when I tried that I basically crippled my neck for several weeks.

Any suggestions?

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