Let number of children who have been vaccinated ONLY against rubella = a
number of children who have been vaccinated BOTH against rubella and mumps = b
number of children who have been vaccinated ONLY against mumps = c

the number of children who have been vaccinated against rubella is twice the number who have been vaccinated against mumps
a + b = 2(b+c)
a = b + 2c

The number who have been vaccinated against both is twice the number who have been vaccinated only against mumps
b = 2c

If 5,000 have been vaccinated against both, therefore b = 5000
Using above equations
c = 2500
a = 10000

IMO C