feet or foot?

If I'm not mistaken, it's - That is a three foot long plank. Yes?
I think it is also - That plank is three feet long. Yes?
And I'm quite sure it's - There are three foot long planks (as in three planks each measuring one foot). Yes?
How about - There are five three feet long planks? (as in five planks each measuring three feet. Should it still be with "foot")
Help.

Yes, it should be "There are five three foot long planks? (as in five planks each measuring three feet." because "three foot long" is an adjective.

Other examples:
"It is a two metre long stick."/"It is two metres long."
"She is a three year old girl."/"She is three years old."
"It is a two litre jug."/"It has a capacity of two litres."

Right. TQ. Now how shall I explain to my students the difference between "That plank is three feet long" and "That is a three foot long plank"? or "Those planks are three feet long" and "Those are three foot long planks"? Aah..it is the explaining that I find difficult.

Yeah, I know what you mean. How to explain, that depends on them I guess.

Thanks all the same.