Question 21
There are 27 transparent boxes, as shown in figure 1. Each layer contains 9 boxes, as shown in figure 2. Some black marbles are placed in the boxes and viewed from different directions. The images are shown in the diagram below. At least how many black marbles are there in these 27 boxes?
Editorial projection convention: a black dot means at least one marble somewhere along that sight line; an empty cell means none. The grids are aligned orthographic views of the same three layers.
Editorial projection convention: a black dot means at least one marble somewhere along that sight line; an empty cell means none. The grids are aligned orthographic views of the same three layers.