If the algebraic sum of deviations of 20 observation measured from 23 is 70, mean of these observation would be

Mean = (sum of observation) / (number of observation)
Sum of observation is given as = 70
Number of observation = 20
∴ Mean = 70/20 = 3.5
But, here observation starts from= 23.
∴ Mean = 23 + 3.5 = 26.5