Answer
See below.
Work Step by Step
a) The mean of $n$ numbers is the sum of the numbers divided by $n$.
Hence the mean: $\frac{85+95+90+85+100 }{5}=91$
b) The deviation is the difference of the data item and the mean.
Hence here the deviations: $85-91=-6,95-91=4,90-91=-1,85-91=-6,100-91=9$
c) The sums of the deviations: $-6+4-1-6+9=0$
