Posted by Anonymous on Tuesday, October 2, 2012 at 7:43pm.
boy's rate = 1/2.5 or 2/5
sister's rate = 1/4
combined rate = 2/5 + 1/4 = 13/20
time at combined rate = 1/(13/20)
= 20/13 hrs or appr 1.54 hrs or 1 hour and 32 minutes
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