Example 5. In the corner of the room, there are several identical cubes piled up. How many of the cubes are not visible?
In the second layer, there is 1 that is invisible,
in the third layer, there are $(1+2)$ that are invisible:
in the fourth layer, there are $(1+2+3)$ that are invisible, in the fifth layer, there are $(1+2+3+4)$ that are invisible, so the total number of invisible ones is $1+3+6+10=20$.