When you add all 8 numbers you'll get 58 then you divide them by 8 you will get 7.25 which is the mean. To find out what is the decrease we need to take out the 30 and add all the other numbers and we get 28. After that you divide 28 by 7 and you get 4. Subtract 4 from 7.25 and you get 3.25.
