Os Números Cardinais ( Cardinal Numbers ):

The numbers 1 and 2 ( and 21,22,31, 32 , 41, 52 etc..) have a masculine form( um, dois ) and a feminine form ( uma, duas ) . Thus , we say um livro ( one book) , dois brasileiros ( two Brazilians ) , and uma mesa ( one table ) , duas brasileiras ( two Brazilians); vinte e um americanos ( twenty-one Americans ), trinta e duas pessoas ( thirty-two people). Pessoa is feminine even if describing a man. 
The same is true for any number that ends in - entos. Therefore we talk about duzentos meninos ( 200 boys) and duzentas meninas ( 200 girls ) ; quinhentos e dez alunos ( 510 students), quinhentas e dez alunas ( 510 students ) etc..
Note, however, that nouns that refer to groups , such as alunos , will appear in the masculine form even if the group has a majority of female members. So, if we have a class with 200 alunas and 5 alunos, we use the masculine form to refer to the whole group: duzentos e cinco alunos.

Now try this one :
1.) 600________________convidados ( guests).
2.) 200________________mesas.
3.) 350 _______________litros de refrigerante ( soda)
4.) 420 ______________latas de cerveja ( cans of beer)
5.) 950 _____________copos ( glasses )
6.) 1001 _____________dias ( days )
7.) 1001 _____________noites ( nights)
8.) 2.000_____________palavras ( words )
9.) 2.000_____________problemas ( problems )

Boa Sorte ! Good Luck !