The brithrate is 0.5 antelopes/year/antelope. The death rate is 0.1/year. The two cannot be equal if the population is stable.
Right.
" The death rate is 0.1/year. The two cannot be equal if the population is stable.
You're given the average lifespan, not the deathrate. You find the deathrate by noting that a stable population is a given and that requires the deathrate = birthrate.