Function for finding SUM combinations

I would not say that there is a single "function" that does this, but a spreadsheet can be programmed to perform this task. Here's how I usually see it set up:

1) Enter your list of numbers in a column
2) Adjacent to this column, enter a list of 1's and 0's. This column will be our "toggle" for including a specific value in the sum.
3) Use the =SUMPRODUCT() function to sum the product of these two columns: http://office.microsoft.com/en-us/ex...in=HP010342656
4) Call Solver. Tell it to set the SUMPRODUCT() cell to the desired value by changing the column of 1's and 0's subject to the constraint that the column of 1's and 0's are binary.
5) To add a constraint that it should only use a certain number of entries, add a cell that sums the column of 1's and 0's (=SUM() function), then add a constraint that this cell should be less than or equal to the desired number of entries.
5) Wait for Solver to find a solution, or decide it is taking too long, or decide that it cannot find a solution.

You will need to recognize that the success rate for this will often be mixed. On the one hand, with 1000 entries to choose from, there are untold millions of possible combinations that Solver will need to try before it decides a solution has been found. Even if a solution exists, it could take some time for it to find a solution. There will also be the question of any possible solution or a specific solution when multiple possible solutions exist. Also the question of will a solution exist at all and how long to you let it try before it should just give up and say there is no solution.