Probably very easy but i'm stupid

The average of three numbers is 57. The average of two of them is 45. What is the third number.

I tried defining each number as a random variable x, y, z and doing

(x+y+z)/3 = 57 and (x+y)/2 = 45 and solving simulataneously but couldn't get it to work.

any ideas?

Quote:

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Originally Posted by IndoorKid

The average of three numbers is 57. The average of two of them is 45. What is the third number.

I tried defining each number as a random variable x, y, z and doing

(x+y+z)/3 = 57 and (x+y)/2 = 45 and solving simulataneously but couldn't get it to work.

any ideas?

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For starters one problem is that there is not a unique answer:

and


Solving the second equation for y gives


Inserting this into the first equation gives:






So there are a range of possible values for y, subject to the restiction that y cannot be negative, and only one score we can predict with certainty: the 81.

-Dan

We assume:

Average of two numbers is 45, so the total of numbers is 45 * 2 = 90
Average of 3 numbers is 57 so the total is 57 * 3 = 171

x + y + z = 171 (1)
x + y = 90 (2)

Subtract (2) from (1):

x + y + z - x - y = 171 - 90
Then, z = 81


Ooops.. didn't know there already was a reply.

thanks heaps guys