Amolecule that can be used as a molecular clock has a neutral mutation rate of one mutation per 20 million years. how many years ago did two species share a common ancestor if the molecules found in these two species differ by a total of six mutations?

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.