The backs of the stickers complete a puzzle in 30 pieces. There are variations where the same fronts of the stickers will have different back puzzle pieces. While not numbered, I have designated them by row (numbers 1 thru 5) and columns (letters A thru F), starting in the upper left corner. Thus the upper left corner puzzle piece will be designated A1, while the lower right corner will be F5.

The puzzle grid is laid out in this fashion:
A1 B1 C1 D1 E1 F1
A2 B2 C2 D2 E2 F2
A3 B3 C3 D3 E3 F3
A4 B4 C4 D4 E4 F4
A5 B5 C5 D5 E5 F5

Upper left corner is A1, lower left corner is A5, upper right corner is F1 and lower right corner is F5.