Mean is average. Multiply total books by the mean to find the total cost:
5x 10 = $50 total.
Subtract the highest and lowest prices to find the total of the remaining 3 books.
50 - 15-6 = 29
The median price was 8, this is the middle number, subtract 8 from 29 to get the cost of the last 2 books:
29-8 =21
Now you need a price lower than the median but higher than the lowest cost so that has to be 7
Subtract 7 from 21 to get the cost of the last book 21-7 = 14
The price of the remaining books is $7 and $14