Question 21
There are 27 transparent boxes arranged as in Figure 1, with 9 boxes per layer as in Figure 2. Black marbles are placed in the boxes and viewed from above, the front and the right as shown. Find the number of marbles.
Occupancy convention: each small box contains at most one marble; aligned marbles overlap as one black dot in a view. A blank projection cell means no marble along that line.
Occupancy convention: each small box contains at most one marble; aligned marbles overlap as one black dot in a view. A blank projection cell means no marble along that line.