Could

Could
 

My sentences as usual:

-These scientists were hard working. They did a lot of things. In that experiment, John__ manage to figure out what would happen to human beings.

Is it correct to say that could is wrong and was able to should be used?

Could is good:)

Thanks.

========

If could is correct, then can I use could here as well?

-Last night, I was in the library working very hard. Later in the night, I got up and went to the shelves. I moved my eyes to the left and could find three useful books.