Mark has completed 5 birdhouses. that is 25% of the number of birdhouses she wants to build. how many birdhouses does she want to build?

Another way to phrase this problem is
5 is 25% of what number?
Let's call the number x and you should know 25%= 0.25

Hint: when you see "is" in these problems you can replace it with an "="

So 5 = 0.25x
Divide both sides by 0.25 to get your answer

The question is basically asking 5 is 25% of what number?
Using a ratio table might help.

 % : birdhouses
25 :  5
100 : ?

Think about what number has to be multiplied to 25 to get 100 and then use that same number (x) and multiply it by 5 to get the total number of birdhouses Mark wants to build.