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考试类型: Mathematics > Standard Multiple Choice

Read the following SAT test question and then click on a button to select your answer.

Cathy likes to wear 2 ribbons, each a different color. If she has 6 ribbons each of a different color, how many combinations of 2 different-colored ribbons can she select to wear?

A. 3

B. 6

C. 12

D. 15

E. 36

正确答案:D

解析:

It may be helpful to number the ribbons 1, 2, 3, 4, 5, and 6. Ribbon 1 can be worn with ribbons 2, 3, 4, 5, and 6, giving 5 combinations. Ribbon 2 can be worn with ribbons 3, 4, 5, and 6, giving 4 more combinations. (Do not recount ribbon 2 with ribbon 1, since that was counted in the first step.) Similarly, ribbon 3 can be worn with ribbons 4, 5, and 6, ribbon 4 can be worn with ribbons 5 and 6, and ribbon 5 can be worn with ribbon 6. The total number of combinations is 5 + 4 + 3 + 2 + 1 = 15.

重点单词   查看全部解释    
recount [.ri:'kaunt]

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vt. 详述,列举,重新计算
n. 重新计算

 
multiple ['mʌltipl]

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adj. 许多,多种多样的
n. 倍数,并联

联想记忆
counted [kaunt]

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vt. 计算;认为 vi. 计数;有价值 n. 计数;计

 

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