# What do we mean by “a double” when we throw two dice?

In my practical assignment, I have this question.

Roll two dice and let:

$A:$ sum of $7$;

$B:$ a double;

Are these two events mutually exclusive?

I can do this part but I am not sure what "a double" means.

What I think it means:

• $(1,1),(2,2),(3,3),(4,4),(5,5),(6,6)$. But as far as I know this is called a "doublet".
• $(1,2),(2,4),(3,6)$ One face is the double of the other.

So someone please tell me what it could be.

• The almost certainly mean that the two faces coincide. I've never heard the word "doublet". – lulu Aug 2 '18 at 15:07
• The first one ... which also includes (6,6) btw – Bram28 Aug 2 '18 at 15:12
• Really @lulu ? en.wiktionary.org/wiki/doublet on the second definition A pair of two similar or equal things; couple. – Isham Aug 2 '18 at 15:20
• @Isham Oh, I didn't mean to suggest that nobody has ever used the term. Just that I have never heard it. And I play a lot of games with dice. – lulu Aug 2 '18 at 15:28
• In many games it's called "doubles" rather than "doublet." Just different terminology for the same thing. – David K Aug 2 '18 at 15:32

As the other answerers have pointed out, a double means that both dice show the same number. "Doublet" or "doublets" is a synonym, see the fifth definition here, or the third definition here. Hence the doubles are (in your notation) $(1,1)$, $(2,2)$, $(3,3)$, $(4,4)$, $(5,5)$, $(6,6)$. You can readily see that these six results have sums $2$, $4$, $6$, $8$, $10$, and $12$, respectively, none of which is 7. Hence the events a sum of 7 and doubles are mutually exclusive.

As an extension of this idea (because I feel that a good answer should give you something else to think about), suppose that you have an $n$-sided die. Let $A$ and $B$ be the events $$A = (\text{the die sum to 7}) \qquad\text{and}\qquad B = (\text{doubles, i.e. both dice show the same number}).$$ Are these two events mutually exclusive? What if we allow the dice to be numbered differently. For example, instead of numbering the die from 1 to $n$, what if we number them from $-n$ to $n$? Are doubles and sums to 7 still mutually exclusive? What if we don't require the numbers to be sequential? What if we don't require the numbering to be integers, but allow any real number? Are the events doubles and sums to 7 still mutually exclusive?

• I think it will be mutually exclusive because there is no way we can get sum 7 from doublets.ohh wait real numbers too. let me think again . – Daman Aug 2 '18 at 15:29
• One instace will be 3.5 one both die. Then it wont be M.E. – Daman Aug 2 '18 at 15:31
• @Damn1o1 Bingo! :) Nicely reasoned. – Xander Henderson Aug 2 '18 at 15:49
• Thanks for putting up the question. I felt like i am getting interviewed :D. – Daman Aug 2 '18 at 15:52
• 1 + 1 = 2 - I'd edit but it would have to be six characters. – tilde Aug 2 '18 at 20:48

It is entirely impossible to know exactly what the author meant. But it seems to me like a relatively safe assumption that they meant that the two dice show the same result.

"But as far as I know its called doublet." And I usually call it a pair. Many things have several different names. That's just the way it is.

Getting the same number on both dice.