Question

Identical cubes are stacked in the corner of a room as shown.

The number of cubes that are not visible are _________.

Answer 1

Draw and observe the diagram:

Let us draw the diagram and observe it,

1. There is no hidden box in the first row.
2. There is $1$ hidden box in the second row, the box in the upper row is sitting on it.
3. There are $3$ hidden boxes in the third row, the boxes in the upper rows are sitting on it.
4. There are $6$ hidden boxes in the fourth row, the boxes in the upper rows are sitting on it.
5. There are $10$ hidden boxes in the fifth row, the boxes in the upper rows are sitting on it.

So, the total number of hidden boxes $=1+3+6+10=20$.

Hence, the number of cubes that are not visible are $20$.