请教怎么做？
１　The operation ? is defined for all nonzero numbers a and b by a ? b = a/b – b/a. If x and y are nonzero numbers, which of the following statements must be true?

I. x ? xy = x(1 ? y)

II. x ? y = -(y ? x)

III. 1/x ? 1/y = y ? x

A. I only

B. II only

C. III only

D. I and II

E. II and III

２　On a certain transatlantic crossing, 20 percent of a ship’s passengers held round-trip tickets and also took their cars abroad the ship. If 60 percent of the passengers with round-trip tickets did not take their cars abroad the ship, what percent of the ship’s passengers held round-trip tickets?

A. 33 1/3%

B. 40%

C. 50%

D. 60%

E. 66 2/3%

第一题答案E.
第二题在PP和OG里面都有相似的题型，有比较详细的思维方式解释.设全部船上的人Y,买了round-trip ticket的人是X,根据题意推出0.4X=0.2Y, 答案是50%, C

OG其实就是GMAT真经，不管什么问题，数学，语法还是逻辑，全部都从OG演变而来，万变不离其宗，全部都可以从OG找到答案。。。