MatricesForKids

Fig 1.

 In Fig.1 above
0) How many properties does each piece have? There is a "correct" relevant answer, but answer this your own way.
1) How many pieces should there be in total to make the set "complete"?
2) How many colors are there?
3) How many shapes are there? 
4) Now answer question 1.
5) How many pieces are missing?
6) Is there any set of the same shape with all the colors?
7) Is there any set of the same color with all the shapes?
8) What are the missing pieces?

Fig. 2

We've categorized the pieces by color, see how many of the above questions you can now answer easily.

Fig. 3

 Now we've categorized the pieces by shape, again, how many of the questions can you now answer easily?

Fig. 4

In Fig. 4, we've structured the information as a matrix, categorizing the pieces simultaneously along the vertical color axis (rows labelled by colors) and along the horizontal shape axis (columns labelled by shape, position in column indexed by color). You should now be able to answer question 8 very easily.