When 40 people used the Atkins diet for one year their mean

When 40 people used the Atkins diet for one year, their mean weight was -2.1 lbs (the negative number here indicating loss). Assume that the standard deviation of all such weight changes is 4.8 lbs and use a 0.05 significance level to test the claim that the mean weight change is less than zero. What kind of test is this?