SOLUTION: ow to make sum 30 using 1,3,5,7,9,11,13,15. any 3 numbers must be used and you can repeat the numbures?

This is a trick question:

To keep from falling for the trick, you must know these 4 facts:

#1. The sum of any EVEN number of any EVEN numbers is always EVEN.
#2. The sum of any EVEN number of any ODD numbers is always EVEN.
#3. The sum of any ODD number of EVEN numbers is always EVEN.
#4. The sum of any ODD number of ODD numbers is always ODD.

Notice that the numbers in the given list are all ODD: 1,3,5,7,9,11,13,15.

Notice that 3 is an ODD number.

Notice that 30 is an EVEN number.

So you are being asked to find an ODD number of ODD numbers that 
has a sum which is EVEN.

But look at fact #4 above.  What you are asked to do is impossible because the
sum of any ODD number of ODD numbers is always ODD, so you will never get 30,
because 30 is an EVEN number! 

So there is NO SOLUTION!

Edwin