cardio wrk out help

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I wanted to know if n e one could answer my question.... i am trying to build ab muscle .... i know you have to do between 25-30 minutes to beginning losing fat.... and no more than 60 min cuz u burn away muscle...... so if i do 20min morning 15min lunch 20min night is that doing okay? Or do i HAVE to do 25-30 consecutive minutes?? THANKS