Question

The average amount with a group of eight members is rs.30. of the newly joined member has rs.65 with him, what was the average amount with the group before his joining the group

Answer 1

average = sum of all observations / total number of observations (1)

Here , now considering the newly joined member ,

total number of observations = 8
average = Rs. 30
sum of all observations = newly joined persons amount + old members amount
But , newly joined persons amount = Rs. 65
So,
sum of all observations = 65 + old members amount

Now, putting these values in (1)
30 = ( 65 + old members amount ) / 8
65 + old members amount = 30 x 8
65 + old members amount = 240
old members amount = 175

Now considering the average before the newly joined member ,

total number of observations = 7
sum of all observations = old members amount
But, old members amount = 175

So, now using (1) ,
average = 175 / 7
average = 25

So, the average amount with the group before his joining the group was Rs. 25