Matching hundreds with gender
In English, numbers do not have a gender. Whether you are counting two hundred men or two hundred women, the words stay exactly the same. When learning Spanish, it is easy to assume that numbers work the same way.
While it is true that most Spanish numbers do not change, the hundreds from two hundred through nine hundred are an exception. Words like doscientos and quinientos actually change their endings to match the gender of the noun they are counting.
When counting masculine nouns, the hundreds end in os. When counting feminine nouns, they end in as. This rule applies to all the hundreds from 200 to 900. It is important to remember that the number 100, cien or ciento, does not change for gender, and neither does mil, which means one thousand.
The lower hundreds are doscientos, trescientos, cuatrocientos, and quinientos. Most of these are formed simply by adding cientos to the base number. The exception is quinientos, which has an irregular base.
The higher hundreds are seiscientos, setecientos, ochocientos, and novecientos. Notice that setecientos and novecientos drop the letter i that you usually see in siete and nueve.
A very common mistake is to forget about gender agreement when the hundred is part of a larger compound number. If you want to say 230 pages, the two hundred part still needs to agree with the feminine word for pages. The treinta part stays the same.
When you use a number between 200 and 900 to count feminine nouns, remember to change the os ending of the hundreds word to as. This applies even when the hundred is part of a larger number.