to work out the mean you add them all up and divide by how many numbers there are:
31+ 28 + 30 + 31 = 120
120 / 4 = 30
to work out the median you order the numbers and then find the middle value
28 30 31 31
therefore the median is 30.5
to find the mode you look for the most common number
which is 31
hope this helps you