Posted by tiffany on Thursday, August 27, 2009 at 10:52am.
Bill's rate = lawn/6
Jim's reate = lawn/9
combined rate = lawn/6 + lawn/9
=5lawn/18
Time for combined effort = lawn/[5lawn/18]
= 18/5 minutes
= 3.6 minutes
Notice that "lawn" canceled, so we could have used any constant for "lawn"
in general for individual times of a,b,c,.. units of time
the time for a combined effort would be
1/[1/a + 1/b + 1/c + ...] units of time
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