# Math Help - Stumped

1. ## Stumped

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

2. Originally Posted by alpina17
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
Dear alpina17,

I don't quite understand your question. What do you mean by "Avg. of 4#'s". Is it average of four numbers in a series?

3. call your 4 numbers: $x1, x2, x3, x4$

the average of these numbers is $\frac{(x1 + x2 + x3 + x4)}{4}$

but you have the answer to this

i.e. $\frac{(x1 + x2 + x3 + x4)}{4} = 27$

=> multiplying across by the $4$ you get: $x1 + x2 + x3 + x4 = 108$

if it's the same set of numbers, $x1, x2, x3$

then the average of these is

$\frac{(x1+ x2+ x3)}{3}$

you also have the answer to this

i.e. $\frac{(x1+ x2+ x3)}{3} = 31$

=> multiplying across by the $3$ you get: $x1 + x2 + x3 = 93$

so therefore $x4$ must be: $108 -93 = 15$

4. 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!)