Solution:
Let x0,…,x199 be variables. Assign the value of +1 or −1 to xi depending on whether the (i+1)st segment of the larger circle (counting counterclockwise) is black or white, respectively. Similarly, assign the value of +1 or −1 to the variable yi depending on whether the (i+1)th segment of the smaller circle is black or white. We can now restate the problem in the following equivalent way: show that
Sj=i=1∑200xiyi+j≥0
for some j=0,…,199. Here the subscript i+j is understood modulo 200.
Now observe that y0+⋯+y199=0 and thus
S0+⋯+S199=i=0∑199xi(y0+⋯+y199)=0.
Thus Sj≥0 for some j=0,…,199, as claimed.