In Mr. Britt's class the distribution of ages of his students has a mean of 115 months and a standard deviation of 3.6 months.

If no students leave or join the class, what will be the mean and standard deviation of the distribution of
student's ages 4 months later?

Using statistical concepts, it is found that the mean will be of 119 months, while the standard deviation will be of 3.6 months, given by option C.

The mean of a data-set is the sum of all values divided by the number of values.The standard deviation is the square root of the sum of the difference squared of each observation and the mean, divided by the number of values.If x is added to all values of a data-set, it is also added to the mean, while the standard deviation remains constant.Mean of 115 months and standard deviation of 3.6 months.4 months later is the same as adding 4 to each observation.Thus, the mean will be of 115 + 4 = 119 months, while the standard deviation remains constant at 3.6 months.

A similar problem is given at link

This is the graph x=1