[spunkets]

"The brithrate is 0.5 antelopes/year/antelope.

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.

[Right Wing Professor]

You're given the average lifespan, not the deathrate.

deathrate = 1/(average lifespan).