Which of the following statement is correct?

I. Average of 10 numbers is 13. If 2 is added to each number, the new average is 15.

II. Average of first 8 prime numbers is 8.75.

III. Average of 15 numbers is 12. The average of first 7 numbers is 10 and average of last 7 numbers is 13. The 8th number is 19.