We only need to consider each number $\pmod{4}$. we have sixteen of each of 0,1,2,3. 0 3 2 1 0 3 2 1 1 2 3 0 1 2 3 0 2 1 0 3 2 1 0 3 3 0 1 2 3 0 1 2 0 3 2 1 0 3 2 1 1 2 3 0 1 2 3 0 2 1 0 3 2 1 0 3 3 0 1 2 3 0 1 2 works for a). For b) let the shapes be P,Q,R,S respectively. When i say 'overlap', it refers to an overlap of two of the shapes P,Q,R,S where they share exactly 3 squares in common. The overlap between P and Q implies that each square is equal to the square 2 directly below it. The overlap between R and S implies that each square is equal to the square 2 to the right. The overlap between P and S implies that each square is equal to the square touching it at the lower right corner. The overlap between Q and R implies that each square is equal to the square touching it at the upper right corner. In fact from these relations a 3 x 3 square must be of the form: Y X Y X Y X Y X Y where X is a number and Y is another. In fact this must extend in all directions so there are only 2 numbers X,Y. This contradicts the fact that 0,1,2,3 must all appear as often as each other.