请教gwd逻辑题

zhaozihan

5-9 Studies in restaurants show that the tips left by customers who pay their bill in cash tend to be larger when the bill is presented on a tray that bears a credit-card logo .Consumer paychologists hypothesize that simply seeing a credit-card logo makes many credit-card holders willing to spend more because it reminds them that their spending power exceeds the cash they have immediately available.

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key: Patrons who are under financial pressure from their credic-card obligations tend to tip less when presented with a restaurant bill on a tray with credit-card logo than when the tray has no logo .

5-20 Shipping Clerk:The five specially ordered shipments sent out last week were sent out on Thursday .Last week ,all of the shipments that were sent out on Friday consisted entirely of building supplies,and the shipping department then closed for the weekend .Four shipments were sent to Truax Constructions last week ,only three of which consisted of buliding supplies .

If the shipping clerk's statements are true  ,which of the following must also be true ?

key :At least one of the shipments sent to Truax Construction last week was sent out before Friday .

如何理解 谢谢

xfi883

5-9：

原文：顾客手头宽裕，所以看到logo后会提醒他们可以多花些钱。

同理，如果顾客有还款压力，那么他们看到logo后只会提醒他们少花钱。

答案中的逻辑有不严密的地方，因此，最好还是采用排除法。

5-20：（简单数学题）

原文大意：

上周四发出了5只船。周五发出的船装的全是建材。上周有4只装建材的船是发往T公司的，其中只有3只是建材。周末不发船。

问：另外一支什么时候发的？

很简单，周五发的都是建材，只有3只发往T公司的是建材，那么肯定有一支不是周五发的。而周末不发船，所以那一支肯定是周五前发的。