Hi guest #22,
How about I try and explain with an easier one.
Let the numbers be 2, 3, 5, and 8 and the magic number that you have to make is 18
Just maybe you might be able to make it just by multiplying things together.
So I will look for all the prime factors of 18
18=2*3*3
we have the 2 and we have one 3
and we have 8 and 5 is we put them together as (8-5) then we have another 3
so one answer would be 18=2*3*(8-5) 
Maybe you would like to try this one. (This is not for big kids who are good at maths)
The numbers are 1,4,2,3,and 6 and the magic number is 27
If you answer this question copy the QUESTION into your post so we know what you are answering :)