I believe the problem ask for s which is the standard deviation. We must recall that the formula for z statistic is stated as:
z = (x – x over bar) / s
Where,
z = z statistic = 2.5
x = sample value or sample score = 102
x over bar = the sample mean or sample average = 100
s = standard deviation = unknown
Rewriting the equation in terms of s:
s = (x – x over bar) / z
Substituting the given values into the equation:
s = (102 – 100) / 2.5
s = 0.8
Therefore the standard deviation s is 0.8