Question

The mean of a set of seven numbers is 81. If one of the numbers is discarded, the mean of the remaining numbers is 78. The value of discarded number is

(a) 98

(b) 99

(c) 100

(d) 101

Answer 1

Given that the mean of 7 numbers is 81. Let us denote the numbers by.

Ifbe the mean of the n observations, then we have

Hence the sum of 7 numbers is

If one number is discarded then the mean becomes 78 and the total numbers becomes 6.

Let the number discarded is x.

After discarding one number the sum becomesand then the mean is

But it is given that after discarding one number the mean becomes 78.

Hence we have

Thus the excluded number is. So, the correct choice is (b).