There are 3 groups of persons — male, female and children. There are 20 males and the number of females and children taken together is 4 more than that of the males. The average weight of males is 54 kg, that of females is 49 kg and that of children is 30 kg. If the average weight of the whole group is 48.25 kg. then what is the difference between the number of females and the number of children?

A

17

B

10

C

7

D

14