A phylogenetic tree for three species, A, B, and C, is shown below. 
A particular DNA region that is shared by all three species is cloned from each species and sequenced. There are about 100 nucleotide substitutions between A and C and between B and C. There are only 20 differences between A and B. The fossil record indicates that the ancestor to A and B diverged from the ancestor to C around 10 million years ago. What is the best estimate of the divergence between A and B based on this information, assuming a constant rate of mutation and the same rate of mutation on all branches?
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