单项选择题

If in an experiment the probabilities of obtaining the values a1，a2，a3，a4， and a5are p1，P2，P3，P4 and P5，respectively，then the expected value is defined as a1p1+a2p2+a3p3+a4p4+a5p5．For the values and their corresponding probabilities in the table above，what is the expected value（）

A. 350
B. 320
C. 300
D. 270
E. 250

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单项选择题
The odds that a certain event will occur is the ratio of the probability that the event will occur to the probability that it will not occur．If the odds that Pat will win a prize are 4 to 3，what is the probability that Pat will not win the prize（）

A.a
B.b
C.c
D.d
E.e
单项选择题
The buyer of a certain mechanical toy must choose 2 of 4 optional motions and 4 of 5optional accessories．How many different combinations of motions and accessories are available to the buyer（）

A. 8
B. 11
C. 15
D. 20
E. 30
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