The number of electrons is 
Explanation:
The total net charge on a negatively-charged object is given by

where:
N is the number of excess electrons
is the charge of a single electron
In this problem, we have
is the overall charge of the object
is the charge of one electron
Solving the equation for N, we find:

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Seems to me that it flies 400 m/s there and 600 m/s back the same distance.
therefore the average of 400 and 600 is 500 m/s. The distance is the same so the normal formula of (d2-d1)/(t2-t1) is applicable.
sorry, it's B, not a made a mistake in the approximation
The answer to your question is C
Because it goes from point A to point C to make one wavelenght