[gbrainy] Add rationale for age puzzle variant

[Jordi Mas <jmas src gnome org>]

commit af25d5e36c97719fc127ad9aa3078a19fee469ad
Author: Jordi Mas <jmas softcatala org>
Date:   Tue May 31 20:46:26 2011 +0200

    Add rationale for age puzzle variant

 data/games.xml |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/data/games.xml b/data/games.xml
index 27bf4c4..f36d19c 100644
--- a/data/games.xml
+++ b/data/games.xml
@@ -59,6 +59,7 @@
 			</variables>
 			<question>John is 46 years old. His son is [difference] year younger than half of John's age. How old is John's son?</question>
 			<question plural ="[difference]">John is 46 years old. His son is [difference] years younger than half of John's age. How old is John's son?</question>
+			<rationale>John's age is nowadays half of John's son age, that is, ([father] / 2) - [difference].</rationale>
 			<answer>[son]</answer>
 		</variant>
 		<variant>