Question 887629
not sure if this is right but i did get an answer.
let's see if it makes any sense.
s = sum of numbers on the balls in the jar.
n = number of balls in the jar.
s/n = 12


if the ball with the number 28 on it is removed from the jar, then the average is decreased by 2.
s-28 = sum of numbers on the balls in the jar after the ball with number 28 on it has been removed.
n-1 = number of balls in the jar after the ball with the number 28 on it has been removed.


you get:
(s-28) / (n-1) = (12-2) which results in:
(s-28) / (n-1) = 10


you have 2 equations to work with.
they are:
s/n = 12
(s-28)/(n-1) = 10


in the first equation, solve for s to get s = 12*n


in the second equation, replace s with 12*n to get:
(s-28)/(n-1) = 10 becomes:
(12*n-28) / (n-1) = 10


multiply both sides of this equation by (n-1) to get:
12*n-28 = 10*(n-1)
simplify to get:
12*n-28 = 10*n - 10


add 28 to both sides of this equation to get:
12*n = 10*n - 10 + 28
simplify to get:
12*n = 10*n + 18
subtract 10*n from both sides of this equation to get:
2*n = 18
divide both sides of this equation by 2 to get:
n = 9


that should be your answer.


since s/n = 12 and n = 9, then s/9 = 12 which makes s = 12*9 = 108


the average is 108 / 9 = 12.


now take 28 away from the sum and take 1 away from n and you get s-28 = 80 and n-1 = 8.


the new average is 80/8 = 10


looks like the answer is correct as far as i can see.


there are 9 balls in the jar originally.