Answer:
Cactus plants are able to survive in drier environment because they are able to store water for long periods.
If dominant allele = p, and recessive allele = q,
and p+q = 1, then:

So if 75% have the p, then p^2 = homozygous dominant

And if the other 25% have the q, the q^2 = homozygous recessive

Now those remaining MUST be the heterozygous, thus 2pq are those:
2pq = 2(.75)(.25) = .375
Therefore homozygous dominant are 56.3% and heterozygous are 37.5%
The population doesn’t grow(increase) anymore and the number of organisms stay at about that point(the carrying capacity).