You're both arguing for no reason, and neither of you is right :o. Warlock, please be more polite. Also, the question was initially formulated imprecisely:

By the way, here's the question: there's a stack of 100 zombies, and 5 tiles away from them is a dragonfly, and 8-10 tiles away are 100 gnolls and 15 lizardmen. What is the probability that the zombies will not go for the gnolls?

The zombies' speed is 4. If the lizardmen are exactly 8 tiles away, then they will naturally rush towards them, because they will reach both the dragonfly and the lizardmen in the same number of rounds. And it's more pleasant for them to attack and block the lizardmen than to attack the dragonfly. The same applies to the gnolls. If the lizardmen are 10 tiles away (i.e., 3 rounds to reach them), then they will go for the dragonfly, trying not to enter the direct line of fire.