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'Odds and Evens' printed from http://nrich.maths.org/
You could start by using the interactivity - what happens when you
pick out two balls from set $A$ $100$ times?
How many times did you get an even total? Were you expecting
Work out all the possible combinations for set $A$, to see if you
can make sense of what happened.
How will you make sure you don't miss any combinations out?