The average age of 45 students in a class in 13 yr, 15 new students with an average age of 18 yr join the class. What is the average age of all the students in the class?

Sum of the age of 45 students = 45 x 13 = 585 yr
Sum of the age of 15 students = 15 x18 = 270 yr
Sum of the age of all students = sum of the age of 45 students + sum of the age of 15 students
= 585 yr + 270 yr
=855 yr
∴ Average age of all students = 855 ⁄ 60
= 14.25 yr