# Number of raisins in a bun

If the average number of raisins in a randomly chosen bun is 9 with a variance of 9, how many buns must be tested if a test with a significance level of 0.05 with 90% probability is gonna reveal than the number of raisins is less than 10?

ATTEMPT:

$$n = \frac{(z_{\alpha} + z_{\beta})^2 \sigma^2}{\delta^2} = \frac{(1.645+1.28)^2 \cdot 9}{1^2} = 77.0006$$

So, at least 78 buns.

Correct answer. At least 82 buns.

What's wrong?