Should I put the papers that I contributed only a little to on my CV When I was a undergraduate student, I worked in a professor's lab doing research about "A". I helped the senior members to do some work they didn't want to do, like designing a website for their demos, or doing some basic calculations. You don't have to have the background knowledge of A to do that work. Then they added me on two papers, though I was always the last one. Afterwards, during my master study, I conducted real research individually in another field "B". Now I am trying to apply for study in an interdisciplinary field that merges fields A and B.

The problem is I know little about A. I am a afraid that the professors in the admission committee would ask me something about A and I definitely can't answer. And they would think I lied, which I didn't. Should I put the papers on my CV?
Your list of publications has to be complete. You should not omit any papers for which you are an author.

If you are uncomfortable with some items on your list, you may add a note on why you happen to be a co-author, but dropping some work is not a good practice. 

In your case, I'd say that there is nothing wrong with these publications: The other authors agreed that your contribution was worth co-authorship and that's usually ok (assuming that the other authors know their field and acted appropriately). If somebody asks about that paper, you can frankly say that you did the calculations but that you can not answer questions about background/other parts or such.