mean = (sum of values) / (number of values)
we know mean is 13 and we are given 8 values. The ninth value is the missing x:
13 = ((sum of 8 values) + x)/9
13 = (12+10+15+12+13+15+11+15+x)/9
13*9 = 103+x
x = 14
The missing value is 14. There are 14 pillows in the last box.
