There seems to be some confusion with this family. I have seen Lemuel Washburn listed as married to Cynthia Rawlins but she died before 1850 (not in 1870) and his second wife was named Nancy.

The 1860 District 56, Mercer Co., Missouri census has:
Lemuel Washburn 56
Nancy 39
Nathaniel 15
Etham 12
Polly 11
Cynthia 11
Rebecca 7
Lemuel 9

The 1860 Morgan, Mercer Co., Missouri census has:
Lemuel Washburn 61
Nancy 48
Mary 22
Lemuel 18
Rebecca 16

The 1850 Vernon, Scioto Co., Ohio census has:
P. Washburn 27
Albina 21
Lemuel 3
Mary E. 1
A. Cable 11

I found a website indicating Cynthia died prior to 1850 (http://freepages.genealogy.rootsweb.com/~sandras/rawlins.htm...) and had 4 children with Lemuel Washburn:
William
Nancy
Permenius
David

Does anyone have any information on Rebecca. She must have married after the 1860 census. With Rebecca and her parents (Lemuel and Nancy) all being from Ohio and her age fits with a Rebecca I am trying to find. Does anyone know her date of birth or who she married?