Data Sufficiency

We can represent this problem as an OVERLAPPING SETS problem: red v. not red, wool v. not wool:



Of the 800 sweaters in a certain store, 150 are red. How many of the red sweaters at the store are made of pure wool?
We can also infer from the stem that 650 are not red:



1) 320 of the sweaters at the store are neither red nor made of pool wool.
We can infer that 330 are wool but not red:



... but we can't infer how many are red wool. Insufficient.

2) 100 of the red sweaters at the store are not made of pure wool.
From this, we can infer that 50 are both red and wool:



This answers our target question, so it's sufficient.

The answer is B.

Overlapping Sets are a common question type on the GMAT, and the double-set matrix is often the easiest way to solve.

For more on how to use the matrix, see:
https://www.beatthegmat.com/at-a-charit ... tml#828495
https://www.beatthegmat.com/among-the-m ... tml#827756
https://www.beatthegmat.com/for-what-pe ... tml#818652
https://www.beatthegmat.com/in-a-produc ... tml#818274
https://www.beatthegmat.com/retailers-q ... tml#809549
https://www.beatthegmat.com/in-a-class- ... tml#802582
https://www.beatthegmat.com/jefferson-s ... tml#708717
https://www.beatthegmat.com/sandwich-t2 ... tml#730500
https://www.beatthegmat.com/ds-question ... tml#677080
https://www.beatthegmat.com/survery-res ... tml#576105
https://www.beatthegmat.com/rainbow-tro ... tml#555603