Statistics Practice Test 1

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