Mary has a pink dress, a blue dress, and a yellow dress. she has a black pair of shoes and a white pair. how many dress and shoe outfits can she make?

There are two ways to solve this. 
The first way is logically. 
Mary can 
-wear a pink dress with black shoes   -wear a pink dress with white shoes
-wear a blue dress with black shoes   -wear a blue dress with white shoes
-wear a yellow dress with black shoes   - wear a yellow dress with white shoes

Count them up, and you'll get six combinations! 

Another way is simply mathematically, which is easier in my opinion. 

3 (different colored dresses) × 2 (different colored shoes) = 6 (combos)

23x 6; 72x24; 12x 4; 9x 3; 3x 1; etc.