Joel gives 1/3 of his baseball cards to his sister. write a fraction that is equivalent to 1/3

All you have to is multiply 1/3 by 2/2 and that would be equivalent to 1/3 or you can multiply 1/3 by any fraction that has the same numerator and denominator such as: 3/3 or 4/4

Equivalent fractions are those that have the same value, but are just expressed differently. You can find equivalent fractions by multiplying or dividing the numerator and the denominator by the same number. For example, multiplying each the 1 (numerator) and the 3 (denominator) of 1/3 by 2 would give you 2/6. 

Some other fractions equivalent to 1/3 include 3/9, 4/12, 5/15, 6/18, 7/21, 8/24, etc.