SOLUTION: 25% of men in a population are married and 30% of females are married. What percentage of the population is married?

It's just a matter of averaging the two percentages.  But that
doesn't explain why.

Assume that there are N men and N women in the population.
That means there are 2N men and women total in the population.

Then 25% of N or 0.25N and married men and 30% of N or 0.3N
are married women.  That's a total of 0.25N+0.3N or 0.55N 
married people.

Now the question is :

0.55N is what percent of 2N?

IS/OF = 0.55N/2N = 0.275 = 27.5%

That's the same as averaging the two percentages: 

Edwin