In a group of students,the mean weight of boys is $65 \ kg$ and the mean weight of girls is $55 \ kg$. If the mean weight of all students is $61 \ kg$,what is the ratio of the number of boys to the number of girls in the group?
- A$2 : 3$
- B$3 : 1$
- C$3 : 2$
- D$4 : 3$
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