Confidence interval: sample mean and sample size

The masses of a particular variety of cherry tomato may be assumed to be normally distributed with a population standard deviation of 6 grams. A sample of size $n$ is randomly selected from a large population of cherry tomatoes and an approximate 90% confidence interval for the population mean of the masses is calculated to be $(11.51, 18.09)$.

Use $\Pr(Z < 1.645) = 0.95$ in your calculations.