请教GMAT数学题一道

题目

请教GMAT数学题一道
29.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 following table.What is the expected number of defective chips in a shipment of 60,000 chips?
Shipment Number of defective chips in the shipment Total number of chips in the shipment
S1 2 5,000
S2 5 12,000
S3 6 18,000
S4 4 16,000
a)14 b)20 c)22 d)24 e)25
所给答案是b.

答案

题目的意思就是新增加的shipment中,number of defective chips in the
shipment与total number of chips in the shipment的比例,与s1,s2,s3,s4的和
的比例是相同的,列式子就是：设新增加的shipment中number of defective
chips in the shipment为X,则X/60000=(2+5+6+4)/(5000+12000+18000+16000),
得到X=20.看看对不,加油!