MAT Exam 2021 Paper

Concept:Use the weighted average formula: class average is the total sum of ages divided by total number of students.Explanation:Let the number of boys be $x$.Total number of students in the class is $x + 20$.Total age of all students = $18(x + 20)$.Total age of boys = $20x$.Total age of girls = $15 \times 20 = 300$.So, the total age of the class is also $20x + 300$.Equate the two expressions:$$18(x + 20) = 20x + 300$$$$18x + 360 = 20x + 300$$$$360 - 300 = 20x - 18x$$$$60 = 2x$$$$x = 30$$Answer:Number of boys = $30$.Correct option: A. $30$.