Brian has 3 basketball cards and 5 baseball cards. what fraction of brian's cards are baseball cards .

The fraction = part/whole, or in this case the # of baseball cards/total number of cards. The total number of cards is 8 because 3 + 5 = 8, and there are 5 baseball cards. So the fraction is 5/8.

Basketball cards = 3
Baseball cards = 5

Total = 3 + 5 = 8

Fraction that are baseball  =  (Number of baseball) / (Total)
 
                                            = 5/8