How to make a value from a variable into a parameter for a function?

Okay, based on my understanding you'd like to make a function average 5 numbers, every 7.5 seconds, all in which should be random. First, let's get a few things started.

1) Creating our function

First thing we're going to do, is create the function that will average the number of numeric inputs we give it. I'll explain this as I go:

2) Declaring your parameters

In this case, we can use a variadic function. A variadic function works the same as any other function, with the addition of being able to take in as many arguments as you want to pass to it. This parameter slot can only be used once, and must be used at the end of your parameter list. Here's an example:

local function Test(...) -- '...' is how you declare a variadic function.
    print(...) -- This will print the amount of arguments we pass
end

Test(1,2,3) -- prints '1', '2', '3'
Test(1,2,3,4,5) -- prints '1', '2', '3', '4', '5'
Test('hello','world') -- prints 'hello' 'world'

Now we have a function that we can pass an unlimited amount of arguments to.

Following the same rules I listed above, I'll go over some invalid examples of declaring variadic functions:

local function Test(..., ...) -- This will error. You can only use this infinite marker once per function, and this would be considered a syntax error.
    print(...)
end

Invalid example #2

local function Test(...,x,y) -- This is also a syntax error. You cannot have arguments after an infinite marker (this defies logic we don't even understand yet)
    print(...,x,y)
end

So that leaves us with creating a variadic function in one of two ways:

a) Declared arguments w/ variadic arguments

local function Test(x,y,...) -- This will work, since we can declare x and y existing before our variadic place holder
    print(x,y,...)
end

Test(1,2,'hi','there') -- Runs the function with x=1, y=2, and ... being 'hi' and 'there' (Note that '...' is a set of unpacked arguments)

To understand what I meant by unpacked arguments, I suggest visiting this: http://wiki.roblox.com/index.php?title=Function_dump/Basic_functions#unpack

b) Completely variadic functions

And this goes back to the first example I showed you, basically looking like this:

local function Test(...)
    print(...)
end

Test(1,2,3) -- And we should know how this works by now...

3) Our function's code

Now we need something for the function to run. In this case, we know we're returning only numbers to the function, which will make this next procedure a bit easier:

First, let's put our arguments into an array (or, list in other terms) when we call the function:

local function AverageNumbers(...)
    local Nums = {...} -- Now our arguments are stored in a list.
end

3) Math and For loops

Now that we have our template down, we can get to the exciting part; our function's logic. We know our array consists of numbers, and we're obviously going to use a for loop to iterate it. Here's how our code should look:

local function AverageNumbers(...)
    local Nums = {...}
    local Total = 0 -- This will be how much our numbers add up to

    -- Numeric for loop time!
    for i = 1,#Nums do -- Iterate through the array's length
        Total = Total + Nums[i] -- Add the current index to the total
    end

    -- Now we divide the total number by how much content our list stores, to get the average.
    Total = Total / #Nums

    -- And finally, return the value to the function call.
    return Total
end

print(AverageNumbers(5,5,5,5,5)) -- Average is 5
print(AverageNumbers(2,4,2,4,2)) -- Average is 2.8

4) Implement the function in your game logic

Now we have the function down, and it's ready to be implemented! Let's apply this to a revised version of your while loop for a final product:

local function AverageNumbers(...)
    local Nums = {...}
    local Total = 0 

    for i = 1,#Nums do
        Total = Total + Nums[i] 
    end

    Total = Total / #Nums
    return Total
end

while wait(7.5) do
    -- Set a list for 5 random values
    local Randoms = {}

    -- Store them via for loop
    for i = 1,5 do
        Randoms[i]  = math.random(1,100)
    end

    -- Return them, but using the unpack function. You can find a document for this function using the link I gave above, in my explanation.
    print(AverageNumbers(unpack(Randoms)))
end

And that's it. If you have any questions, let me know. If you don't know much about how for loops work, I suggest checking these sites:

http://wiki.roblox.com/index.php?title=Loops#unpack http://www.lua.org/pil/4.3.4.html

Hope it helped!