4. What is the value of the two-digit number x？

　　（1） The sum of its digits is 4.

　　（2） The difference of its digits is 4.

　　Considering （1） only， x must be 13， 22， 31， or 40. Hence， （1） is not suffici

　　ent to determine the value of x.

　　Considering （2） only， x must be 40， 51， 15， 62， 26， 73， 37， 84， 48， 95， or 5

　　9. Hence， （2） is not sufficient to determine the value of x.

　　Considering （1） and （2） together， we see that 40 and only 40 is common to th

　　e two sets of choices for x. Hence， x must be 40. Thus， together （1） and （2）

　　are sufficient to uniquely determine the value of x. The answer is C.

　　5. If x and y do not equal 0， is x/y an integer？

　　（1） x is prime.

　　（2） y is even.

　　（1） is not sufficient since we don't know the value of y. Similarly， （2） is

　　not sufficient. Furthermore， （1） and （2） together are still insufficient sin

　　ce there is an even prime number——2. For example， let x be the prime number

　　2， and let y be the even number 2 （don't forget that different variables can

　　stand for the same number）。 Then x/y = 2/2 = 1， which is an integer. For al

　　l other values of x and y， x/y is not an integer. （Plug in a few values to v

　　erify this.） The answer is E.

　　6. Is 500 the average （arithmetic mean） score on the GMAT？

　　（1） Half of the people who take the GMAT score above 500 and half of the peo

　　ple score below 500.

　　（2） The highest GMAT score is 800 and the lowest score is 200.

　　Many students mistakenly think that （1） implies the average is 500. Suppose

　　just 2 people take the test and one scores 700 （above 500） and the other sco

　　res 400 （below 500）。 Clearly， the average score for the two test-takers is n

　　ot 500. （2） is less tempting. Knowing the highest and lowest scores tells us

　　nothing about the other scores. Finally， （1） and （2） together do not determ

　　ine the average since together they still don't tell us the distribution of

　　most of the scores. The answer is E.