2/3 of the 60 are either cows or pigs, so the number we are concerned with here is 40.

A. This tells us that there are at least 2x cows for every x pig.
3x=40
x=13.3333
Since you cant have a third of an animal we have to round down and say x=13

The thing to remember here is the wording in the statement. At least is telling us that this is simply a range and not a firm number. What we get from this is that Pigs<=13 and Cows are >=27. That does not give us the information to answer the question.

B. This is clearly not enough. It could have 20 pigs, 30 pigs, etc...

Now combining the 2 we know that

12<P<=13

The only way to fill the inequality is for the number of pigs to equal 13. Thats a firm number and now we can also see that there are 27 cows. This is sufficient for solving the question.