The total ratio representing the number of children in the family is 5+7 = <<5+7=12>>12
From the total ratio of children in the family, 5/12 represent the number of boys, meaning that the number of boys in the family is 5/12*180 = <<5/12*180=75>>75
If the boys are given $3900 to share, each boy receives $3900/75 = $<<3900/75=52>>52
#### 52