suppose it take Cindy x hours, so each hour she weed 1/x portion
it takes jim 2x hours, each hour he finishes 1/(2x) portion
together they finish 1/x +1/(2x) portion per hour, so for 6 hours that is
[tex] \frac{6}{x} + \frac{6}{2x} =1
\frac{12}{2x} + \frac{6}{2x} =1
\frac{18}{2x} =1[/tex]
2x=18
x=9
