Bill:

I find it hard to believe that this question belongs in "Early Childhood Education," but here goes:

To find the z-score you need to also know the mean and the standard deviation of the target population:


z = \frac {x- \mu} {\sigma}\


where:
x = raw score to be standardized,
\mu = mean of the population, and
\sigma = the standard deviation of the population.

You've told us that x = 78, but not what either \mu or \sigma are.