Here are answers + commentary to the number puzzle I posted yesterday, which was, fill in the digits:
ABC +DEF GHI
OR prove it's impossible.
I originally moderated the answers but have now gone back and published them all. My conclusion is that the folks who read this blog are way smarter than I am.
So it is indeed possible, and the general consensus from comments is that there are 336 solutions.
Solutions generally fell into 3 categories:
Proving it's impossible?
One interesting catch is that you need a carry bit (2 digits that add to over 10). In my case, (218+439 = 657), that was the 8+9=17. You can easily prove that you need a carry-bit with simple number theory (such as by inspecting even + odds). There are 5 odd numbers in 1...9 and 4 even numbers. The carry bit gives you an extra odd number (a one) in the mix. (See Korn1699's comment for further explanation). Once I noticed that, I used it to prune the search space in my adhoc search.
This sort of thing is another cute example of "impossible" vs. "insufficiently clever". If you forget about carry bits and just look at the numbers straight up, you can easily come up with a flawed proof that this is impossible.