Subject :IMO Class : Class 8
The mean weight of 120 students in a class is 56 kg. If the mean weight of boys and that of the girls in the class are 60 kg and 50 kg respectively, then the number of boys in the class are_____.
A72
B38
C64
D57
Ans 1:
Class : Class 8
Let us say that the total sum of weight is x
x/120 = 56
x = 6720
Now, let us say that the number girls is z
We know that the sum of the weight of the girls will be the sum of weight of the boys subtracted from the total sum.Let us say that the sum of weights of boys is y.
So now girls total weight sum will be : 6720-y/z = 50
6720-y = 50z
6720 = 50z + y
6720 - 50z = y(Equation 1)
Total sum of weight of boys will be
y/120 - z = 60
y = 7200 - 60z(Equation 2)
Subtracting both equations(1 and 2) :
y-y = 7200-60z - 6720z + 50z
0 = 480-10z
10z = 480
z = 48
Number of girls is 48.So, the number of boys will be 120-48 = 72