Well, the way I make it is
C= 40000000M therefore (Cx3.142857143/2=r) the Radius is 62857142.86 M
C= 40000000 + 10 is C = 40000010M therefore the radius is 62857158.57 M
62857142.86 - 62857158.57 is 15.71428571M.
Your answer of 1.6 M would be correct if the wire were only 1 M longer.
Or have I missed it completely?
err...C = 2*pi*r, so r = C/(2*pi). Dunno why you're multiplying by pi, and pi = 3.14159... by the way, not 3.142857...