I think when the statement says : "it sold different number of copies on any two days of the week", it meant that no number of books sold on any particular day is the same.
Now here's my answer:
Assume that 11 copies were sold on Friday, that must mean all the other days must have lower and different numbers of copies sold (since Friday is the second greatest). Now try to pick number for the other 5 days (e.g. 6,7,8,9,10) and we can see that no matter what number we pick for those days, the number of copies sold on Saturday must be equal or greater than 39 in order to add up to a total of 90.
The same thing goes for the case of less than 11 copies were sold on Friday, the number of copies sold on Sat must be equal or greater than 39.
However, if the number of copies sold on Friday was greater than 11, the number of copies sold on Sat can both be 39 and greater (40, 12, 11, 9, 8, 7, 3) or 38 and below (37, 15, 10, 9, 8, 7, 4).
Thus we can rephrase the question as: Was the number of copies sold on Sat equal or greater than 39 (in which case the statement would be insufficient) or below 39 (in which case Friday would have to be greater than 11 so that the total number of books add up to 90).
Statement 1:
One of the "less" day is 8 copies => insufficient since from the 3 number picking examples above we can see that Friday can be greater, or equal to 11.
Statement 2:
Sat = 38. Sufficient because now we know Friday would have to be greater than 11 in order for all the days to add up to 90
Answer: B
