Question

Identical cubes are stacked in the corner of a room as shown below. The number of cubes that are not visible are _____.

Answer 1

Let's look at the number of cubes that are not visible in each row.

first row = 10 cubes are not visible
second row = 6 cubes are not visible
third row = 3 cubes are not visible
fourth row = 1 cube is not visible
Fifth row = 0 (In fifth row only one cube is there which is visible.)

$\text{Hence, Total number of cubes that are not visible}=10+6+3+1+0=20$