Eggs

$3~hens~~~~~~~~~~~~~~~3~days~~~~~~~~~~~~~~~\Rightarrow 3~eggs$ $\Rightarrow 3~hens~~~~~~~~~~~~~~~3 \times 2=6~days~~~~~~~~~~~~~~~\Rightarrow 3 \times 2=6~eggs$ $3 \times 2=6~hens~~~~~~~~~~~~~~~6~days~~~~~~~~~~~~~~~6 \times 2=\boxed{12}~eggs$

If 3 hens give 3 eggs in 3 days then 3 hens will give 6 eggs in 6 days and 6 hens will give 12 eggs in 6 days.