The mean weight of 150 students in a class is 60 kg. Total weight of 150 students = 150 × 60 = 9,000 kg Number of boys = x Number of girls = (150 - x) Mean weight of boys = 70 kg Total weight of boys = 70x Mean weight of girls = 55 kg Total weight of girls = 55 (150 - x) = 8,250 - 55x So, 70x + 8,250 - 55x = 9,000 15x = 9,000 - 8,250 15x = 750 x = 50 So, Number of boys = 50