[求助]prep1数学二道

Shipments      number of defective chips in the shipment   total number of chips in the shipment

s1                  2                                                                    5000

s2                  5                                                                    12000

s3                  6                                                                    18000

s4                  4                                                                    16000

A computer chip manufacturer expects the ratio of the number of defective chips to the total number of chips in all future shipments to equal the corresponding ratio for shipments s1,s2,s3, and s4 combined, as shown in the above. what is the expected number of defective chips in a shipment of 60000 chips?

A14 B20 C22 D24 E25

惭愧，没看明白题目在问什么，请高人指点！

If a coin has an equal probabilty of landing heads up or tails up each time it is flipped, what is the probability that the coin will- land heads up twice in 3 consecutive flips?

第一题我选B.它的意思说，以后的船上defective chips和total chips的比值等于前四条船s1 s2 s3 s4各自defective chips和total chips的比值之和。也就是：x/60,000=(2+5+6+4)/(5,000+12,000+18,000+16，000)

算出来x=20

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第二题，抛硬币。正面和反面朝上的概率均为1/2。问连续抛三次中两次为正面朝上（其实正反面都无所谓）的情况发生的概率。

三次中两次正面朝上，C(3,2)=3;

这两次正面朝上，(1/2)^2=1/4;

还有一次反面朝上，1/2;

所以概率为C(3,2)*(1/2)^2*1/2=3/8