prep3数学-38，55，154

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38、If r and s are positive integers, is  r/s an integer?

 

 

(1)  Every factor of s is also a factor of r.

(2)  Every prime factor of s is also a prime factor of r.

为何选A?（2）为何不成立？

55、Malik's recipe for 4 servings of a certain dish requires 1 cups of pasta.  According to this recipe, what is the number of cups of pasta that Malik will use the next time he prepares this dish?b

 

 

(1)  The next time he prepares this dish, Malik will make half as many servings as he did the last time he prepared the dish.

 

 

(2)  Malik used 6 cups of pasta the last time he prepared this dish.

答案是C,没搞明白

154、The sum of positive integers x and y is 77.  What is the value of xy ?

 

 

(1) x = y + 1

 

 

(2) x and y have the same tens digit.

（2）为啥也成立？

[此贴子已经被作者于2008-7-29 17:37:06编辑过]

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不好意思，我的数学挺差的，而且没查到这些题以前的讨论帖子（大概由于方法错误），所以请多多指教

～～

jh1904

1. Set s=2^3*5^2, r = 2*5, r/s is not an integer. If s=2*5, r=2^3*5^2, then r/s is an integer. Condition 2 not sufficient.

2. condition 1 tells us that Malik will use 1/2 of what he used last time.

    condition 2 tells us that Malik used 6 cups last time.

With condition 1 and 2, we can calculate that Malik will use (1/2) * 6 cups.

Ans : C

3. x+y = 77, x and y have the same 10s digit so x and y can only be 38 and 39, so xy =38*39.

 (If x =37 then y =40)

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了了～～谢谢