求救一道GWD数学题

eliasfan

Q19:

In a certain region, the number of children who have been vaccinated against rubella is twice the number who have been vaccinated against mumps.The number who have been vaccinated against both is twice the number who have been vaccinated only against mumps.If 5,000 have been vaccinated against both, how many have been vaccinated only against rubella?


A.2,500
B.7,500
C.10,000
D.15,000
E.17,500
Answer: 是C
但不知是怎么来的，请求NN们指导

sdcar2010

The number who have been vaccinated only against mumps: x
The number who have been vaccinated against both: 2*x
The number of children who have been vaccinated against rubella: 2*(x + 2*x) = 6*x

2*x = 5000 --> x = 2500

The number of children who ONLY have been vaccinated against rubella = The number of children who have been vaccinated against rubella - The number who have been vaccinated against both
= 6*x - 2*x = 4*x = 10,000

eliasfan