Question

the average age of an adult is 40 years 12 new students with an average age of 32 years join the class thereby decreasing the average of the class by 4 years the original strength of the class was

Answer 1

The average age of an adult=40 years. let number of adults be x. So sum of all ages of adults =40(x)= 40x . Sum of all 12 students of average age of 32 years=32(12)=384 . Sum of ages of adults and students =40x+384 .As average drops by years , new average of this group=36 years . Number of members in this group=x+12 => 36= 40x+384/x+12
=> 36(x+12) =40x+384 => 40x-36x=432-384=48 =>x=48/4= 12 . So the original strength of the class is 12 .Hope it helps you...