At a sleep over, six friends were talking about the number of hours of sleep they usually get each night. Five of them said: 12,9,4,8, & 7 hours. The sixth friend said, “ the average of our sleep hours is 9, so how many hours do I sleep ?

So average of a set of values is the sum of given values divided by the number of values

So, in this case the given values are 12, 9 , 4 , 8 ,7 and x (letting the 6th friend's time be x)

so the average will be:

(12 + 9 + 4 + 8 + 7 + x)/6 = 9

40 + x = 54

x = 54 - 40

x = 14 hours