Blank dice

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Here is a probability brain teaser that I remember from a math for elementary ed class I tutored.


When you roll two six-sided fair dice, the total number of dots displayed can be anything from 2 to 12. However, some sums are more likely than others. Now suppose you have two six-sided fair dice, with no dots on any of the faces (they are blank). Armed with only a marker pen and your puzzle-solving prowess, how can you draw dots on the dice so that each possible sum will appear with equal probability?

Some solutions are trivial, for example you could just leave all the sides blank so that the only possible sum (zero) appears with 100% probability. But thinking about the trivial solutions may well be the first step to finding better solutions in which each dice has a different number on each side.