help me

babylu

狒狒笔记上的一道题弄不明白，哪位高人给指点一下？
In an insurance company.,each policy has a paper reocrd or an electric record,or both of them.60 percent of the policies having incorrect paper record have incorrect electric record and 75 percent of the policies having incorrect electric record have incorrect paper record.3percent of all the policies have both incorrect paper and incorrect electric records. If we rancomly pick out one policy, what is the probability that it is one having both correct paper and correct electric record?

babylu

他给的答案是：    p                 /p
                   e     x%          (96-x)%          96%(推出）
                  /e   1%(推出） 3%(已知）     4%（推出）
                                        （占/e75%)
   则答案是（96%-x%+3%)*60%=3%      x=94              但我没能看明白

qiqi

我的解法请参考:为清晰多用一些说明:
1.设总的policy数为A.设总的having incorrect paper record 数为X,设总的the policies having incorrect electric record 数为Y.
则根据题意有3%A=X+Y-(both correct paper and correct electric record),我们可以设both correct paper and correct electric record为M, 即 3%A=X+Y-M ,另有M=60%X,同时M=75%Y。解出3%A=（100/60+100/75-1）M，此题所求的为M/A，所以M/A=3/200。和你给的答案不太一样。