1. **State the problem:** Find the z-score for $x=78$, given mean $\mu=70$ and standard deviation $\sigma=4$. 2. **Formula:** The z-score formula is $$z=\frac{x-\mu}{\sigma}$$ which measures how many standard deviations $x$ is from the mean. 3. **Substitute values:** $$z=\frac{78-70}{4}$$ 4. **Calculate numerator:** $$z=\frac{8}{4}$$ 5. **Simplify fraction:** $$z=\frac{\cancel{8}}{\cancel{4}}=2$$ 6. **Interpretation:** The value 78 is 2 standard deviations above the mean. **Final answer:** $$z=2$$