100 customers bought 200 books. How many purchased only 1 book each?

1. None of the customers purchased more than 3 books.

In this case you can think of a scenario in which each of the customer purchased 2 books, or another in which a few purchased 3 while some others purchased 1 or 2. You therefore cannot have a unique value for how many purchased JUST 1.

INSUFFICIENT.


2. 20 customers purchased only 2 books each. This accounts for 40 books. Therefore 80 of the customers purchased 160 books.

This is similar to our original question, and again we cannot identify how many people purchased exactly 1 book. It could have been that 79 of them purchased 1 book each and the last one purchased 81, or any other combination.

INSUFFICIENT.


Both 1 and 2 together:

So now, out of those 100 customers, none of them purchased more than 3. This means that they either purchased 1, 2 or 3 books. But we also know that 20 of them purchased 2. So the 80 remaining purchased either 1 or 3. Let's say x people bought 1 book. So 80-x ppl bought 3 books

x*1 + (80 - x)*3 = 160 .... we can solve for x

SUFFICIENT

Pick C.

Combining both we can find the solution,

from stmt 1 and stmt 2,

20 customers purchased exactly 2 books each(ie) 40 books.

remaining 80 cutomers purchased 160 books.

from stmt1 ,none of them purchased more than 3 books.

So 80 customers purchased 1 or 3 books.

If x customers purachsed 1 book ,80-x purchased 3 books.

x*1+(80-x)*3=160

x=40

Sufficient.