This question is from a book.
In 1970 the average number of children per family in a certain town was 3.8. In 1980 a random sample of 40 families had a total of 144 children. At the 5pc level of significance is there evidence to conclude that the mean number of children per family had decreased?
I want to use this formula: $z=\frac{x-\mu}{\frac{\sigma}{\sqrt{n}}}=\frac{3.6-3.8}{\frac{\sigma}{\sqrt{40}}}$
The book says "Yes" and gives a z-value answer of -0.2108. I could not figure out the standard deviation from the problem. Is it there?