In this data sufficiency question, the following is asked: "If each of the 20 bolts of fabric on a shelf is either 100 percent cotton, 100 percent wool, or a mixture of cotton and wool, how many bolts are cotton and wool mixtures?"

(1) Of the 20 bolts, 18 contain some wool and 14 contain some cotton.
(2) Of the 20 bolts, 6 are 100 percent wool.

The correct answer is A. In the solution, the first statement is sufficient because: (14 - n) + n + (18 - n) = 20. The "n" is defined as the number of bolts that are cotton and wool mixtures. Can you explain this part in detail, please? Specifically, I don't know how "some wool" and "some cotton" from statement one are translated into the equation above.

Picture a Venn diagram here.
The left circle intersects the right circle.
The left circle represents those bolts containing cotton, the right circle represents those containing wool, and the intersection represents those containing both.
"some cotton" means that the bolt might be 100% cotton OR a blend of cotton and wool. (Same goes for "some wool" here. )
Then they use the formula we use to solve these types of problems.

I'll use a slightly different variation and see if it clarifies the issue for you:

Total = cotton + wool - both (we subtract "both" because it is counted twice; once with cotton and once with wool. "cotton" here means all bolts that contain cotton - those that are 100% and those that are a blend. Same goes for "wool" as well.)
20 = 18 + 14 - both
20 = 32 - both
both = 12 (this is the number of bolts that are a blend.)

Veritas Help