Answer: Option D.
Let x be the original average and n be the number of students. Let x
1, x2, x
3 ,. . . .. x
n be the weights of n students respectively. Therefore,
x
1, x2, x
3 ,. . . .. x
n / n = x
x1, x2, x3 ,. . . .. xn = nx...............(1)
Now,according to 1st condition we have
x1, x2, x3 ,. . . .. xn + 50/ n+1 = x+1
x1, x2, x3 ,. . . .. xn = (n+1)(x+1) - 50..............(2)
Further,according to 2nd condition we have
x1, x2, x3 ,. . . .. xn + 50+50/ n+2 = x+1.5
x1, x2, x3 ,. . . .. xn = (n+2)(x+1.5) - 100..............(3)
Solving (1) and (2), we get
x + n = 49…………….. (4)
Solving (1) and (3), we get
4x + 3n = 194 ……………. (5)
Now, Solving (4) and (5), we get
n = 2, x = 47
So, the original average weight (in kg) of the class = 47
Alternate Solution :
Let x be the original average and n be the number of students.
From the first increase in the average,
we get 50 – x = n + 1
From the second increase in the average,
we get 100 – 2x = 1.5 (n + 2)
Solving, we get the value of x = 47.