The frequency distribution of the compressive strength of 20 concrete cube specimens is given in the table.



f(MPa) Number of specimens with

compressive strength equal to f

23 4

28 2

22.5 5

31 5

29 4



If $\mu$ is the mean strength of the specimens and $\sigma$ is the standard deviation the number of specimens (out of 20) with compressive strength less than $\mu -3\sigma$ is ________.

1. 0