The average age of three boys is 15 years and their ages are in proportion 3:5:7. What is the age in years of the youngest boy? Choose the correct choice from the following choices.
A) 8 B) 9 C) 7 D) 5 E) 4
3x + 5x + 7x = 45   x =3   3x = 9   Answer:9
<result>B</result>
The average age of three boys is 15 years and their ages are in proportion 3:5:7. What is the age in years of the youngest boy?