OG11-PS 63

Kath1985

63 搜尋不到 所以畫圖求牛牛們指導 感謝
[attachimg=951,445]75936[/attachimg]
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 ta
ble above.  What is the expected number of defective chips in a shipment of 60000 chips ?
A 14
B 20
C 22
D 24
E 25
答案為B
OG的做法是 (2+5+6+4) /5000+12000+18000+16000=n/60000
但為何是全部加起來的不良率比上60000的不良率  ?
為什麼不能用其中一列比上60000呢?  例:用S4中的  4:16000=n:60000

Kath1985

請問黃色部分的思路哪裡錯了呢?