29. The price of a full-fare coach ticket from Toronto to Dallas on Breezeway Airlines is the same today as it was a year ago, if inflation is taken into account by calculating prices in constant dollars. However, today 90 percent of the Toronto-to-Dallas coach tickets that Breezeway sells are discount tickets and only 10 percent are full-fare tickets, whereas a year ago half were discount tickets and half were full-fare tickets. Therefore, on average, people pay less today in constant dollars for a Breezeway Toronto-to-Dallas coach ticket than they did a year ago.

Which one of the following, if assumed, would allow the conclusion above to be properly drawn?

(A) A Toronto-to-Dallas full-fare coach ticket on Breezeway Airlines provides ticket-holders with a lower level of service today than such a ticket provided a year ago.

(B) A Toronto-to-Dallas discount coach ticket on Breezeway Airlines costs about the same in constant dollars as they did a year ago.

(C) All full-fare coach tickets on Breezeway Airlines cost the same in constant dollars as they did a year ago.

(D) The average number of coach passengers per flight that Breezeway Airlines carries from Toronto to Dallas today is higher than the average number per flight a year ago.

(E) The criteria that Breezeway Airlines uses for permitting passengers to buy discount coach tickets on the Toronto-to-Dallas route are different today than they were a year ago.

the answer is B. I really don't understand.

today 90 percent of the tickets are sold at a discount, while last year only 50% were.

Conclusion: Therefore, on average, people pay less today in constant dollars for a Breezeway Toronto-to-Dallas coach ticket than they did a year ago.

If you want to compare, at least you need to have the same base, i.e., the same amount of cost for a discount ticket. otherwise, you can not compare. e.g. in constant value, if this year's ticket after discount is $60, while last year's one was $20, then, let's do a little calculation here:

Assume each year there are 100 people and the full-fare ticket cost $100.

Then this year's cost on average, in costant dollar: (90 x $60 + 10 x $100)/100 = $64

Last year: (50 x $20 + 50 x $100)/100 = $60

Obviously, on average, this year's ticket costs more than last year's one. That is not what the argument wants.

Therefore, we need the assumption like (B) A Toronto-to-Dallas discount coach ticket on Breezeway Airlines costs about the same in constant dollars as they did a year ago.

but the passage has already told us that "The price of a full-fare coach ticket from Toronto to Dallas on Breezeway Airlines is the same today as it was a year ago, if inflation is taken into account by calculating prices in constant dollars". That makes the full-fare ticket price in constant dollars this year the same as previously.

Do we still need another compare basis?

其实这一题无论 constant dollar 或 inflation 都只是用来干扰你的. 不用去理它. 但是 "on average" 是关键

这一题是在问: 在总票数不变, 今年特价票比去年多的情况下, 全价票价格和去年相同, 如果平均票价要比去年低的话需要有什条件: 当然是特价票价也要差不多和去年相同.