For question 4:

1.  the area of our galaxy would be pi*r^2  =3.14*(400000)(400000) square light years
    which is about 5 billion square light years

2.  In that area there are 1000 civilizations uniformly distributed so that means
    there is civlization per  5 million square light years.

3.  The radius of a circle that gives an area of 5 million square light years is
    about 4000 light years


Note:  to think about this problem, it really helps to make a diagram of a circle with
       some dots in it uniformly spaced