Nancy can hula hoop for 10 minutes.  Casey can hula hoop 3 minutes less than Nancy.  Morgan can hula hoop three times as long as Casey.  How long can Morgan hula hoop?
Casey can hula hoop 3 minutes less than Nancy so she can hula hoop for 10-3 = <<10-3=7>>7 minutes
Morgan can hula hoop three times as long as Casey, who can hula hoop for 7 minutes, so she can hula hoop for 3*7 = <<3*7=21>>21 minutes
#### 21