Now Dot. It's not really a math problem you see. One would think that the figures I gave below would give you the proper answer, but, they don't. Surprise! The only way to know for sure is to put the N scale inside the actual larger cars. There is space left over of course since we are not going to chop up any cars to cram the entire space. So, are we getting any closer to the correct figures? Only whole cars were considered in the prize winning answers.
By the way, just for kicks I did the math on fitting the HO car into the O car and it came out as 3.719 cars. However, in truth, only one HO scale car would fit into the O scale car, with some space left over.
I guess it all has to do with the shape of the items. No more hints. You're on your own.
B-)