Average number of girls in all three school together is:

Let number of girls in School ‘A’ is ‘x’. Number of boys in School ‘C’ = 1.20 × x = 1.2x Total number of students in School ‘C’ = 1.2x/0.6x = 2x Number of boys in School ‘B’ = 0.8x + 40 Number of girls in School ‘B’ = 0.8x + 40 – 120 = 0.8x – 80 Total number of students in School ‘B’ = 0.8x + 40 + 0.8x – 80 = 1.6x – 40 So, 1.6x – 40 + 2x = 840/0.6 = 1400 Or, 3.6x = 1440 Or, x = 400 Number of girls in School ‘A’ = 400 Total number of students in School ‘A’ = 840 + 400 = 1240 Number of boys in School ‘C’ = 1.2x = 1.2 × 400 = 480 Total number of students in School ‘C’ = 2x = 2 × 400 = 800 Number of girls in School ‘C’ = 0.8 × 400 = 320 Let number of boys in School ‘B’ = 0.8 × 400 + 40 = 360 Number of girls in School ‘B’ = 0.8 × 400 – 80 = 240 Total number of students in School ‘B’ = 360 + 240 = 600 Desired Average = (400 + 240 + 320)/3 = 320