The mean deviation is the average of the absolute differences between each value in a set of values and the mean of the set. To calculate the mean deviation of the given set of values, we first need to find the mean of the set. The mean is calculated by adding all the values and dividing by the number of values. For the given set of values, 3, 5, 8, 11, 12 and 21, the mean is (3 + 5 + 8 + 11 + 12 + 21) / 6 = 10.

Next, we need to find the absolute differences between each value and the mean. The absolute differences are: |3 - 10| = 7, |5 - 10| = 5, |8 - 10| = 2, |11 - 10| = 1, |12 - 10| = 2, and |21 - 10| = 11.

The mean deviation is calculated by adding all the absolute differences and dividing by the number of values. So, for this set of values, the mean deviation is (7 + 5 + 2 + 1 + 2 + 11) / 6 = 4.67, which can be rounded to 4.7.