The average age of three boys is $15$ $year.$ If the ratio of their ages is $3:5: 7,$ what is the age of the oldest boy in $year$?
The average age of three boys is $15$ $year.$ If the ratio of their ages is $3:5: 7,$ what is the age of the oldest boy in $year$?
- A
$7$
- B
$14$
- C
$20$
- D
$21$
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