what do you mean by harmonic mean ???

Harmonic mean is used to calculate the average of a set of numbers. Here the number of elements will be averaged and divided by the sum of the reciprocals of the elements. The Harmonic mean is always the lowest mean.

Harmonic Mean Formula :
Harmonic Mean = N/(1/a₁+1/a₂+1/a₃+1/a₄+.......+1/a_N)
where 
              X = Individual score 
              N = Sample size (Number of scores)