I hate to say it but he may be right. We've been assuming that all the antelope born, lived; but if each one has 5 progeny in ten years and they all live, there's no way the population can stay at 24,000 antelope. So if a lot were killed to hold the population constant, their births add to the number of mutations even though they don't show up as a change in the population.
Thank God I'm a recognized numbers dolt; otherwise I'd be so embarrassed!
Looking back at the original problem, I see the question is "how many mutations would enter the gene pool"; this makes the problem impossible to solve, since there's no way of knowing whether the large number of offspring killed are killed before or after reaching breeding age.