Find the mode of all digits used in writing the natural numbers from 1 to 100.

Find the mode of 10, 12, 11, 10, 15, 20, 19, 21, 11, 9, 10.

Mode of certain scores is x. If each score is decreased by 3, then find the mode of the new series.

Observations of a raw data are 5, 28, 15, 10, 15, 8, 24. Add four more numbers so that mean and median of the data remain the same, but mode increases by 1.

If the mean of a set of observations x₁, x₂, ...., ...., x₁₀ is 20. Find the mean of x₁ + 4, x₂ + 8, x₃ + 12, .... ...., x₁₀ + 40.