Finding valid and distinct integers for digits S, E, N, D, M, O, R and Y in the equation SEND + MORE = MONEY is a classical constraint programming problem. Here's the slightly modified version of the sample program at Wikipedia constraint programming entry for SWI-Prolog using clp library:
:- use_module(library('clp/bounds')). sendmore(Digits) :- Digits = [S,E,N,D,M,O,R,Y], Digits in 0..9, S #\= 0, M #\= 0, all_different(Digits), 1000*S + 100*E + 10*N + D + 1000*M + 100*O + 10*R + E #= 10000*M + 1000*O + 100*N + 10*E + Y, label(Digits).
Here's what happens when we load the code and run
?- consult('money.pl'). % library(clp/clp_events) compiled into clp_events 0.00 sec, 2,948 bytes % library(clp/bounds) compiled into bounds 0.03 sec, 90,992 bytes % money2.pl compiled 0.03 sec, 91,812 bytes Yes ?- sendmore(X). X = [9, 5, 6, 7, 1, 0, 8, 2] ; No
So, 9567 + 1085 = 10652.
AFAI remember, there was a similar question asked to Google's candidate software engineers, which was
WWW + DOT = COM.
Well, you now know the answer ;)
I'll include both examples for my PySWIP Python package which enables to query SWI-Prolog from Python.