Uniform and identical meter sticks are stacked, so that 20cm of each stick extends beyond the meter stick beneath as shown in figure. How many maximum numbers of sticks can be placed one over the other without making them to fall over?

The given metre sticks are identical in nature.
The centre of gravity of each stick is located at its mid-point as shown in the fig.

Let, n be the maximum numbers of sticks that can be placed one over the other without making them to topple. 
Let the edge O of the lowest stick S₁, which is on the floor be the reference point.
The distances of centre of gravity of sticks S₂, S₃, S₄......S_n are x₂ = -30cm, x₃ =-10cm, x₄ = 10cm .........x = -30 + 20(n-2) = (20n -70)cm respectively.

Let m be the mass of each stick.
The position of centre of gravity of the sticks S₂, S₃, S₄......S_n from the O is,

The condition for sticks not topple is that the position of CG of sticks S₂, S₃, S₄......S_n should lie on left from edge of the S₁.

i.e.,    X ≤ 0

     10n - 50 ≤ 0

      n ≤ 5

      n ≤ 5

Hence, at the maximum, five sticks can be stacked one over other.