Jenny takes 3 hours to sand a picnic table; Laila can do the same job in \(\dfrac{1}{2}\) hour...

$$Rate\ =\ work\ done\ per\ unit\ of\ time$$
$$Rate\ =\ \frac{workdone}{time}$$
$$Jenny's\ rate\ =\ 1\ table\ in\ 3\ hours=\frac{1}{3\ }table\ per\ hour$$
$$Laila's\ rate\ =\ 1\ table\ in\ \frac{1}{2}hours=1\div\frac{1}{2\ }=\ 1\cdot\frac{2}{1}$$
$$=\ 2\ tables\ per\ hour$$
$$Working\ together\ =>$$
$$Total\ rate\ =>\ \frac{total\ work\ done}{time}$$
$$\frac{Combined}{total\ rate}\ =\ \frac{1}{3}+\frac{2}{1}=\frac{1+6}{3}=\frac{7}{3}$$
$$Two\ of\ them\ can\ sand\ \frac{7}{3\ }picnic\ table\ per\ hour$$
$$Time\ taken\ for\ the\ 2\ of\ them\ to\ sand\ 1\ picnic\ table\ $$
$$=\ time\ taken\ =\frac{total\ workdone}{total\ rate}$$
$$time\ =\ 1\div\frac{7}{3}$$
$$=\ 1\cdot\frac{3}{7}=\frac{3}{7\ }hours$$
$$Answer\ =\ D$$