The mean book rating will decrease
Add all the numbers together:
5 + 5 + 6 + 7 + 7 + 7 + 7 + 8 + 8 + 8 + 9 + 9 + 10 + 10 + 10 + 10 + 10= 136
divide the total with the total amount of numbers
136/17 = 8
Now add all the numbers again WITH the one
5 + 5 + 6 + 7 + 7 + 7 + 7 + 8 + 8 + 8 + 9 + 9 + 10 + 10 + 10 + 10 + 10 + 1= 137
Divide the total amount with the amount of numbers there are
137/18 = 7.61
7.61 < 8
The mean book rating will decrease by 0.39
hope this helps