Across-section of a tree trunk shows 20 dark-colored rings and 20 light-colored rings. what is the likely age of this tree?

The number of rings is equal to the number of years the tree has lived. The light and dark colors of the rings only indicate the warm and cold seasons they have been through. For a year, there's summer and winter so you don't count it as 40. So, the tree is 20 years old. 

Uh...selective permeability? Some things can pass through the membrane whilst other things can't diffuse freely and passively (like larger, polar molecules-like that bug). Maybe lol.