Aptitude question and answer on Average

Question : The average age of boys in the class is twice the number of girls in the class. If the ration of boys and girls in the class of 36 be 5:1, what is the total of the ages (in years) of the boys in the class ?
A.    380
B.    342
C.    372
D.    360

Answer : D.  360

Explanation:
Number of boys = $\bigg(36 \times {5 \over 6}\bigg) =30$
Number of girls = 6.
Average age of boys= (2 x 6)=12 years.
$\dot{..}$ Total age of boys = (30 x12) years= 360 years