我的gwd数学错题集 更新中

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有的时候GWD数学找不到解析，我只能把我错的，总结一下放上来。自己想不明白的请教大家，自己走过的冤枉路希望可以帮助其他小伙伴找到答案。NN请不要笑我的silly answer。GWD-1
2.Acertain roller coaster has 3 cars, and a passenger is equally likely to ride inany 1 of the 3 cars each time that passenger rides the roller coaster.  If a certain passenger is to ride the rollercoaster 3 times, what is the probability that the passenger will ride in eachof the 3 cars?

A.    0

B.    1/9

C.    2/9

D.    1/3

E.    1

正确答案C，错选B。错误原因：（1/3）乘（1/3）乘（1/3）=1/9.但实际上ACB和ABC 都可以所以是2/9. 正确思路：树状图，共27个可能结果，六种满足题目要求ABC ACB, BAC BCA, CBA CBA. 故6/27=2/9

3. Ifx and y are positive, is 4x > 3y?

(1)   x > y - x

(2)   x/y < 1

A.Statement (1) ALONE is sufficient, but statement (2) alone is notsufficient.

B.Statement (2) ALONE is sufficient, but statement (1) alone is notsufficient.

C.BOTH statements TOGETHER are sufficient, but NEITHERstatement ALONE is sufficient.

D.EACH statement ALONE is sufficient.

E.Statements (1) and (2) TOGETHER are NOT sufficient.

正确答案E，错选B。错误原因：以为x/y<1,代表x<y,但不代表x>3/4y. 正确思路：stt1推得x>.5y不充足,stt2推得x<y.不充足。 1+2得0.5y<x<y,则2y<4x<4y.但不能验证4x和3y关系，故E。

9. Agardener is going to plant 2 red rosebushes and 2 white rosebushes.  If the gardener is to select each of thebushes at random, one at a time, and plant them in a row, what is theprobability that the 2 rosebushes in the middle of the row will be the redrosebushes?

A.   1/12

B.   1/6

C.   1/5

D.   1/3

E.   1/2

正确答案B，错选D.依然是概率和排列组合问题. 正确思路：树状图，共24个可能结果，4种满足题目要求w1r1rww2 w1r2r1w2, w2r1r2w2 w2r2r1w1. 故4/24即1/6.

10. Aset of numbers has the property that for any number t in the set, t + 2 is inthe set.  If –1 is in the set, which ofthe following must also be in the set?

I.             -3

II.            1

III.           5

A.          Ionly

B.          IIonly

C.          Iand II only

D.          IIand III only

E.          I,II, and III

正确答案D，错选E.错误原因：读完题认为，奇数或偶数在此数集里。已知-1在，即奇数集则-3，1，5都在。正确解析：G老头都很保守，已知-1在，只能推出比-1打的奇数在数集里，不能逆推因为-1在，所以-3也在。-1有可能是数集的第一个数。

17. InTown X, 64 percent of the population are employed, and 48 percent of thepopulation are employed males.  Whatpercent of the employed people in Town X are females?

A.   16%

B.   25%

C.   32%

D.   40%

E.   52%

正确答案B,错选A。错误原因：审题不清，答案为计算完整。正确答案：64%-48%=16%，16%/64%=1/4，即25%。

27. Ifn = 811 – 8, what is the units digit of n?

A.   0

B.   1

C.   4

D.   6

E.   8

正确答案C，错选E.错误原因：蒙的。正确解析：unit digit是个位，问各位的数字是什么。要分别求8的一次幂个位是8，二次幂的个位是4，3的是2，4的是6，5的是8，6的是4，所以规律是8/4/2/6，11次幂正好是2，12-8=4.即C。

30. Joannabought only $0.15 stamps and $0.29 stamps. How many $0.15 stamps did she buy?

(1)   She bought $4.40 worth of stamps.

(2)   She bought an equal number of $0.15 stampsand $0.29 stamps.

A.Statement (1) ALONE is sufficient, but statement (2) alone is notsufficient.

B.Statement (2) ALONE is sufficient, but statement (1) alone is notsufficient.

C.BOTH statements TOGETHER are sufficient, but NEITHERstatement ALONE is sufficient.

D.EACH statement ALONE is sufficient.

E.Statements (1) and (2) TOGETHER are NOT sufficient.

正确D，错误为C。正确解释：试数。stt1 试数可得，即使两个未知变量。stt2也是试数可得。所以E。

GWD-2

12: There are 11 women and 9 men in a certainclub.  If the club is to select acommittee of 2 women and 2 men, how many different such committees arepossible?

A.   120

B.   720

C.   1,060

D.   1,520

E.    1,980

正确E。排列组合问题。从n个数字中选择k个（上n下k）=n！/k!(n-k)!.此题中先从11个女生中选两个为（11 2），从9个男生中选两个（9 2）两者相乘，即E。

13:Aninteger greater than 1 that is not prime is called composite.  If the two-digit integer n is greater than20, is n composite?

(1)   The tens digit of n is a factor of the unitsdigit of n.

(2)   The tens digit of n is 2.

A.Statement (1) ALONE is sufficient, but statement (2) alone is notsufficient.

B.Statement (2) ALONE is sufficient, but statement (1) alone is notsufficient.

C.BOTH statements TOGETHER are sufficient, but NEITHERstatement ALONE is sufficient.

D.EACH statement ALONE is sufficient.

E.Statements (1) and (2) TOGETHER are NOT sufficient.

正确A，错选E。错误原因：stt 1的逻辑推理不对。我的错误想法是因为25他是composite但是2不是5的因数，所以stt 1不对。这个想法是错误的，因为题干是正推，不需要找出特例证明题干错误，只需要顺着验证题干正确就可以了。24，36，48，都是符合的所以stt1是对的。故A。

26. Thereare 8 books on a shelf, of which 2 are paperbacks and 6 are hardbacks.  How many possible selections of 4 books fromthis self include at least one paperback?

A.   40

B.   45

C.   50

D.   55

E.    60

正确D。还是排列组合。两种方法。1）正推。两个p全中乘以六个h中2，加上2个p中1乘以6个h重3=（2 2）*（6 2）+（2 1）*（6 3）=15+40=55。2）逆推。总的（8 4）减去全部是h的（6 4）=70-15=55。

29. If1050 – 74 is written as an integer in base 10 notation, what is thesum of the digits in that integer?

A.  424

B.  433  

C.   440

D.   449

E.    467

正确C。 这题很虎人，但是看了解析发现居然被虎到了。10^2=100-74=26，10^3=1000-24=926,10^4-74=9926, 10^5-74=99926.故10^50-74=9999....926.共（50-2）即48个9+2+6=440。

780一次过

Mark一下！