The mean age of a group of students is 15 years. When the age of a teacher, 45 years old, is added to the age of the students, the mean of their ages becomes 18 years. Find the number of the students in the group
A. 9 B. 7 C. 42 D. 15
Correct Answer: A
Explanation
Let the number of students = x Total age of the students = 15x Total age including age of the teacher = 15x + 45 Mean of their ages; (15x + 45)/(x + 1) = 18 15x + 45 = 18(x + 1) 15x + 45 = 18x + 18 18x - 15x = 45 - 18 3x = 27 x = 9
