How would I go about finding the multiples of x?

You can simply have an arithmetic check on the number you want to check if its a multiple or not.

The arithmetic operation is called "modulus", essentially what it does it checks the remainder of the division between 2 numbers and if it is 0 it means it is a multiple of that number ( the divisor ).

Here is an example with code:

function returnIfMultiple(a, multiple)
    if ((a % multiple) == 0) then
        print(a .. " is a multiple of " .. multiple .. " !");
    else
        print(a .. " is not a multiple of " .. multiple .. " !");
    end;
end;

As you can clearly see you give this function a number and the multiple you want to check with, in your example you want to check if a number is multiple of 5. You would simply do the mathematic operation 10%5 for example and that would return you 0 implying that 10 is indeed a multiple of 5. If you try to do the same operation with other number such as 11, the remainder that will be returned by the modulus operation will not be 0 therefore it is not a multiple of 5.

Make sure to ask any questions if you have any.

:)