Identical cubes are stacked in the corner of a room as shown below. The number of cubes that are not visible are _____.
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Solution
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.)
Hence, Total number of cubes that are not visible=10+6+3+1+0=20