Please forgive me if I have this in the wrong forum, but I am 'stuck' my 12yr old daughter came to me with this & I want to run & hide..
Avg. of 4#'s is 27
Avg. of 3#'s is 31
What is the 4th #?
Any help seriously appreciated
call your 4 numbers: $\displaystyle x1, x2, x3, x4 $
the average of these numbers is $\displaystyle \frac{(x1 + x2 + x3 + x4)}{4}$
but you have the answer to this
i.e. $\displaystyle \frac{(x1 + x2 + x3 + x4)}{4} = 27 $
=> multiplying across by the $\displaystyle 4 $ you get: $\displaystyle x1 + x2 + x3 + x4 = 108 $
if it's the same set of numbers, $\displaystyle x1, x2, x3 $
then the average of these is
$\displaystyle \frac{(x1+ x2+ x3)}{3}$
you also have the answer to this
i.e. $\displaystyle \frac{(x1+ x2+ x3)}{3} = 31 $
=> multiplying across by the $\displaystyle 3 $ you get: $\displaystyle x1 + x2 + x3 = 93 $
so therefore $\displaystyle x4 $ must be: $\displaystyle 108 -93 = 15 $
Excellent! believe it or not that's what I did; 108 - 93
but being honest I had no idea why I was submitting 15,
Now that you have explained, thank you very very much
maybe my daughter will not look at me as a 'd'oh...(well probably asking too much there!)