Statement 1 can't be sufficient info to answer the questing that's being asked. Since 8 books were sold on Thursday, 8 books were sold on Monday, Tuesday, Wednesday, and Sunday. So, a total of 40 books were sold on thos 5 days. We know that the number of books sold on Friday must be greater than 8 since they're the second greatest of the copies sold each day that week. What if 9 books were sold on Friday? Then this would mean that 41 books were sold on Saturday, which, by the way, still fulfills the statement that the greatest number of copies were sold on Saturday. Hence, Statement 1 can't be sufficient.

Statement 2 isn't sufficient on its own but what if we combine statements 1 and 2 to answer the question that's being asked to us?

As I explained above, a total of 40 books were sold on the 5 days of the week. If 38 books were sold on Saturday, the day with the greatest number of copies sold, then the number of copies sold on Friday must've been less than 38. 40+38=78 and 78+9<90, 78+10<90, and 78+11<90. This means that there must've been greater than 11 copies sold on Friday.
The answer's C.

Actually, B is sufficient. I will try to find the explanation.

I n attement 1, why you sya that 8 books were sold in each day ? the stem says that the store sold different number of books each day.

Yeah, I saw where I made my mistake. I need to slow down when I do these data sufficiency problems.

An explanation for the problem can be found here:

https://www.urch.com/forums/gmat-data-su ... ore-s.html

heshamelaziry wrote:Store S sold a total of 90 copies of a certain book during the seven days of last week, and it sold different numbers of copies on any two of the days. If for the seven days Store S sold the greatest number of copies on Saturday and the second greatest number of copies on Friday, did Store S sell more than 11 copies on Friday?

(1) Last week Store S sold 8 copies of the book on Thursday.
(2) Last week Store S sold 38 copies of the book on Saturday

This problem has been discussed before. I can't find its location. But, I remember I couldn't understand it well. Could someone enlighten me more ? Maybe I will get it.

If you wanna minimize something, maximize everything else, and vice versa. Take constraints into account.

(1) Thursday day's sale 8, the sum of remaining = 82.

friday n sat are the two greatest.
friday > 8, which is thursday's. So, we got a minimum number for friday.

Insufficient on its own.

(2) all but sat = 90-38 = 52.

Find two lists one with minimal variation and another with large variation.

first, larger variation:
----------------------
1,2,3,4,5,fri = 52
fri = 62-15 = 37

second, minimum variation:
-----------------------
find average, spread around it.
52/6 = 8.66
The list contains even number of members.
Spread around 8.5, or 8,9
8,9
6,7,8,9,10,11 = 51
bump up 1
6,7,8,9,10,12

friday's min = 12.

12 <= fri <= 37. Sufficient.