Answer: 20.2 mph
Step-by-step explanation:
Given
Train travels 10.1 miles in half an hour i.e.
Distance 
time 
we know, 
Insert the values

It depends on what you draw. So I'd like to say C, because you could pull up all the white marbles the first few draws, or none the first few and every other marble. It really depends.
The missing side length, n equals 7.