Dicey

When you roll 2 fair dice, is the total more likely to be odd or even?

Odd Even They are both equally likely

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17 solutions

Jose Mazariego
Dec 9, 2018
1 2 3 4 5 6
1 2 3 4 5 6 7
2 3 4 5 6 7 8
3 4 5 6 7 8 9
4 5 6 7 8 9 10
5 6 7 8 9 10 11
6 7 8 9 10 11 12

Note, you can count up even and odd sums. Thus 18 even suns, 18 odd sumes. So 50/50 chance

The probability to obtain the sum 7 is the highest or 6/36, therefore the answer is odd

Rene Valdes Asiain - 2 years, 6 months ago

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What? Your reasoning is completely wrong.

Krishna Karthik - 2 years, 6 months ago

this is wrong reasoning

Khalil Khemiri - 2 years, 6 months ago

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You're wrong. This is a sample space and this solution is very valid.

Krishna Karthik - 2 years, 6 months ago

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I disagree, Krishna. Look at it this way: odd+odd = even, even+even= even, odd+even = odd. This gives you the ratio that the likelihood of the roll of the dice to be 2:3. So, the answer is even.

Siri Chennareddy - 2 years, 2 months ago

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@Siri Chennareddy Lol that is so ridiculous!

Chris Arsenault - 2 years ago

Why? Is there any other way?

Satya *** - 2 years, 5 months ago

Actually all the sums are not equally likely to occur. When we see the sums we can notice that 7 occurs more. Then probability becomes same for both even and odd.

Tama Dd - 2 years, 6 months ago

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This is not by sum, but by pair of dice. Each pair of dice result is equally likely.

Orlando Moreno - 2 years, 5 months ago

This is how I did it. I used a sample space (two way table) like the one above. Good job.

Krishna Karthik - 2 years, 5 months ago
Geoff Pilling
Dec 3, 2018

Whatever you roll on the first die, there is a 50/50 chance that the second roll will match it in parity (odd or even). If the parities match, the result will be even. Otherwise, the result will be odd. Therefore, there is a 50/50 chance their sum will be odd. (And, a 50/50 chance it will be even)

Moderator note:

As multiple people have been tripped up by this:

While it is true that there are more even sums than odd in the span from 2 to 12, they're not all equally likely to occur. For example, 7 is more likely than any other result with two dice (you can get it in 6 ways: 1-6, 2-5, 3-4, 4-3, 5-2, 6-1). You can check Jose Mazariego's answer further down for more detail.

I thought of an answer that I was pretty confident in before I read your explanation: rolling two dice could result in any number from 2 to 12. In the range of possible outcomes there are 5 odd results and 6 even results. I thought, therefore, that there would be an overall higher probability of getting an even result.

Spika Rooni - 2 years, 6 months ago

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This is what I thought also.

kevin scott - 2 years, 6 months ago

My first answer was this too. The only possible "totals" are 2 to 12 (there's multiple different combinations that can be rolled... each with the same chance, to obtain those "totals")... but what the "totals" can be are still 2 to 12

And to quote Spika..... "In the range of possible outcomes there are 5 odd results and 6 even results."

Sean Torg - 2 years, 6 months ago

With two dice the total cannot be '1' . There are only eleven other total six of which are even.

Jeff Furner - 2 years, 6 months ago

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Indeed, but not all numbers are equally likely. For example, 7 is more likely than any other result with two dice (you can get it in 6 ways: 1-6, 2-5, 3-4, 4-3, 5-2, 6-1). You can check Jose Mazariego's answer further down for more detail.

Jason Dyer Staff - 2 years, 6 months ago

So if 7 is the most likely combination, how can it be that 7, being an odd number does not make the likelihood of the odd number higher?

James Brown - 2 years, 5 months ago

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Because the numbers with the next highest probabilities are 6 and 8, and you need to add up all the probabilities and compare.

Geoff Pilling - 2 years, 5 months ago

the question is not asking “what is the likelihood of the next roll being ‘even’ or ‘odd’ if the first roll is ‘even’?”, it is asking “is the sum of two fair dice thrown more likely to be ‘even’ or ‘odd’?” the answer to this is ‘even’ by a count of twelve ‘even’ sums to only nine ‘odd’ sums.

the question can be broken down to “if you have any two sets of numbers with an equal amount of even and odd numbers in each set what is the likelihood that two numbers drawn at random (one from each set) have an even sum:

1st # | 2nd # | cardinality of sum —————

even + even = even even + odd = odd odd + odd = even

*note that odd/even is the same as even/odd, and should not be counted (we don’t have a second even/even or a second odd/odd)

Tommy Hugo - 2 years, 5 months ago

Geoff, I like the fact that your proof doesn't utilize the fairness of the first die. In other words, the result hold even if only one die is fair.

Peter Byers - 2 years, 5 months ago
Ralph Gatan
Dec 10, 2018

The thing is we need to have number of combinations which is 6^2(32) as there six different numbers. Now watch and observe closely. E=even and O=odd
1+1 E 1
1+2 O 2
1+3 E 3
1+4 O 4
1+5 E 5
1+6 O 6
2+2 E 7
2+3 O 8
2+4 E 9
2+5 O 10
2+6 E 11
3+3 E 12
3+4 O 13
3+5 E 14
3+6 O 15
4+4 E 16
4+5 O 17
4+6 E 18
5+5 E 19
5+6 O 20
6+6 E 21
You might see why it's only 21 but actually it is 36 because some of them are just their backwards. These are
2+1
3+1
4+1
5+1
6+1 and etcetera. Let me give you another one example.
1 - 1 2 3 4 5 6
2 - 1 2 3 4 5 6
3 - 1 2 3 4 5 6
4 - 1 2 3 4 5 6
5 - 1 2 3 4 5 6



6 - 1 2 3 4 5 6

 6 6 6 6 6 6 = 36 equal!!!!!

Using your method, it will certainly be much more tedious to answer the follow-up question below, right?

When you roll 3 fair dice, is the total more likely to be odd or even?

Pi Han Goh - 2 years, 6 months ago

There are only 11 possible sums where 2 fair dice are thrown and the total shown calculated - from 2 to 12 inclusive. 6 of these are even numbers and 5 are odd numbers. Surely the chance of an odd number total is less than the chance of an even number?

Is this not correct

Colum McLoughlin - 2 years, 6 months ago

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I made the same mistake at first, but the 11 sums are not equally likely! Look at probabilities of each of the sums and then add the probabilities of the evens. Then you'll see that even and odd are indeed equally likely.

Stefan Trost - 2 years, 6 months ago

No because there are 6 x 6 = 36 possible outcomes, you have to count all of the them not just the unique additions...

Mike Massen - 2 years, 6 months ago

What's interesting is that the chance of odd or even from one die is 50% each but, when you sum (total) the numbers from 2 dice you have a summation that favours Even because:-

A. Even plus Even is always Even,

B. Odd plus Odd is always Even,

C. Even plus Odd is always Odd,

However, when you run all the combinations the chance of the sum being odd or even is exactly 50%. What this suggests is one can be misled by the skew of A. and B. giving Even when it's only C. which results in Odd. Of course the quantities of A, B & C are important, if one ignores that then easily misled for a quick answer - like I was :-(

Mike Massen - 2 years, 6 months ago

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A. Even plus Even is always Even,

B. Odd plus Odd is always Even,

C. Even plus Odd is always Odd,

D. Odd plus Even is always Even, There are two more but they are inconsequential. I made the same mistake.

Joane Hasi - 2 years, 6 months ago

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I think U meant to write D. Odd plus Even always Odd. The other two being the ordering of A and B hence where they balance the total is 50/50

Mike Massen - 2 years, 5 months ago
Kenneth Wada
Dec 14, 2018

The correct solution is ... IT DEPENDS! If we assume that the question is about two fair dice ... each being D6 ... then both ODD and EVEN are equally likely... This is the correct result for a PAIR of EVEN sided dice However, if the question is about two fair dice ... each being a D5 ... then EVEN is more likely than ODD. ... This is always the correct result for a PAIR of ODD sided dice. There is NO STATEMENT here other than a PAIR... A PAIR explicitly means that both dice have the same number of faces. However; the question did not explicitly state how many faces ... other than to show a cartoon with a pair of D6's

"It depends" 🖖 though more I thought about it how are we defining 'fair' and in regards to what?
If we just mean the dice will fairly roll each side the same percent then odd sided success are fair game.
If the fairness is that the 'dice' are to be 'standard regular D6 (inclusive of former sense with no improper weighting) then we do maths and get 50/50. If though we take it at face value if they are fair in the sense of the results will be 50/50 then the the dice themselves depends on meeting the result criteria and by definition will always be 50/50 because that's how we selected them. If fairness is personified (think muse like Justice) and has foresight to know if there is a superior reason for odd (or even) to win out and directs fate accordingly then odd would be a more accurate winner and with fairness abiding to ensure fairness rules out (or conversely even - though biased personally to 3,5,9 being superior and more real then other numbers).

My vote is the mythological reality so going to triple down on even numbers getting too much credit and go with odd numbers becoming more real in the 4th dimension hypercube sense.

Kind of what's in my pocket riddle logic

Me Bromkovsky - 2 years, 5 months ago
Stanislav Spassov
Dec 14, 2018

The problem is only concerned with parity (odd vs even) of the sum. The parity of the sum is completely determined by the parities of the individual rolls, and their specific values do not matter. Instead of six-sided dice rolls, this observation allows us to think in terms of two-sided coin throws.

In fact, we can use this simplification regardless of the dice specifics (number of sides, (un-)fairness), as long as the "heads vs tails" probabilities of the imagined coins match the "odd vs even" probabilities of the original dice. For standard, fair six-sided dice, we can think of fair coins.

A fair coin does not care which outcome we call heads, and which one we call tails. All that matters is that there are two outcomes and that they are both equally likely. Considering this underlying symmetry in each individual coin throw, and how each one independently "toggles" the end result ("parity of the sum") in the same 50/50 way, the answer cannot possibly be "it's more likely that the sum is odd than even" (or vice-versa), since it never mattered which option we called "odd" and which "even" in the first place. This means both outcomes for the sum must be equally likely, regardless of the number of dice rolled/coins thrown.

Jake Deschamps
Dec 13, 2018

The chances of the first die being odd are 50% and the chances of it being even are 50%. The same goes for the second die. We can list out the possibilities much like we could for an experiment involving flipping two fair coins. While we do this, we can list the outcome of each roll.

EE (Even + Even = Even)

EO (Even + Odd = Odd)

OE (Odd + Even = Odd)

OO (Odd + Odd = Even)

The outcome is odd in 2 out of 4 cases, so the probability of getting an odd sum is 1/2.

Note that this technique covers the sum of any two fair dice so long as each die has an equal balance between odd and even sides. Our six sided die has three odd sides and three even sides. We could apply this solution to rolling two fair twenty sided dice with the numbers 1-20 on them, as each die has ten even sides and ten odd sides.

We could even apply this to stranger, unfair dice as long as each die has a 50% chance of rolling an odd number and a 50% chance of rolling an even number.

Oon Han
Dec 9, 2018

Without loss of generality, let us assume the first dice roll is even. Then we need to find the probability of the next dice being odd and the probability of the next dice being even, since even + even = even and even + odd = odd.

But, since there are as many odd numbers as even numbers in the sample space of 1 to 6, they should have the same probability of showing up.

Thus, the total is equally likely to get an odd as an even.

Krishna Karthik
Dec 9, 2018

If you draw a two way table to represent the sample space, you will see that there are 18 squares that contain odd numbers, and of course, 18 squares for even numbers, as the complement of odd is even. Therefore the probability to get an odd total the same as the probability of getting an even number.

There are 36 possible outcomes, each equally likely. Half of the outcomes are even and half of the outcomes are odd. Therefore, evens and odds are equally likely.

results = Tally[Flatten[Table[i + j, {i, Range[1, 6]}, {j, Range[1, 6]}]]] gives {{2, 1}, {3, 2}, {4, 3}, {5, 4}, {6, 5}, {7, 6}, {8, 5}, {9, 4}, {10, 3}, {11, 2}, {12, 1}}

Hmmm, can you adapt this code such that you're able to solve the following question too?

When you roll 1000 fair dice, is the total more likely to be odd or even?

Pi Han Goh - 2 years, 6 months ago

I could. I won't. A proof by induction is simpler. For n=1, half of the numbers are even and half are odd. If true for some number m, is it true for m+1? Yes, if the die being added to the sequence is even, then the even half is still even and vice versa; otherwise, if the die being added to the sequence is odd, then the odd half becomes even and vice versa. In either case, the even split is not changed. Q.E.D.

The reason for not writing the code is that the code would take a very long time to run as there are 1416610262383486172379625252491522441664047183091019132232354743214061\ 8947596486436347661333869287260068907949302029484915942402681211620694\ 5980466178442955122207931033129805495915371609590530279406241175980034\ 1750301572269742817615560036226312856759029951177668659286207437632823\ 2990325101248680123776914576482815095784568122986221890411837737570098\ 8646133420909727564696614882161768944653880284167683384953269896751180\ 8722276738459611135130495786902527380297828178373192996646821057922983\ 0069556698928937342508988340792335737744719376598506908977135291983117\ 7226482691779471546576975170749934415155268398870734001917974451537602\ 2169572326825500613404406250310071013420041460769697675783700291138902\ 3284338696251543694980946202137938610119300450795091488653253649628649\ 410789376 cases to be examined.

A Former Brilliant Member - 2 years, 6 months ago

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I thinki'ts fair

Stephen Peter Miranda - 2 years, 6 months ago

This is a computation using fair coins, which is a reasonable simulation of even and odds on a fair die. This uses 1 for heads and 0 from tails. It adds the head each tuple combination and counts by summing the heads and then reduces that to a even or odd determination by reducing the sum modulus 2. I retained the timing in seconds for each computation. and curve fit the running times in seconds on my computer.

FindFit[Table[ First[Timing[ Log[2, Plus @@ Table[Mod[Plus @@ t, 2], {t, Tuples[{0, 1}, n]}]]]], {n, 25}], E^(a x + b) + c, {a, b, c}, x]

The fit is E^(0.716967962881 x + -14.4048401877) - 0.0089040378436

E^(a x + b) + c /. {a -> 0.7169679628806168 , b -> -14.40484018771176 , c -> -0.008904037843599283`, x -> 1000} is

1.31609540927*10^305

The age of the universe in seconds is only about 4*10^17 seconds.

A Former Brilliant Member - 2 years, 6 months ago
Maria Vergara
Dec 16, 2018

They are both equally likely because... Exist equally amount odd number and even number in the dice.

Rajat Ranjan Jha
Dec 15, 2018

There are two ways to get an even total. Either both die outcomes are even, or both are odd. The no. of ways both outcomes are even are 3×3=9 ; and that for both odd outcomes are 3×3=9 also; as there are three odd, and three even outcomes on both dice. Thus, total favorable outcomes is 9+9=18; and same is the case for odd totals. Thus both cases are equally favorable

Jorge Ruvalcaba
Dec 15, 2018

Each dice have 3 odd and 3 even numbers so 50/50 chance.

Ana X Gss
Dec 12, 2018

Each dice has 3 even and 3 odd numbers so when you role them there is 50/50 chance of it being even or odd

Becuse if you count the fact that every number squared will equal an even number, then if you add up all of the numbers it will equal an odd number. Then if you double that you'll get an even number, then you'll have a 50/50 chance of rolling odd or even.

Connor Lane
Dec 11, 2018

You know that 50% or the time one will be odd and 50% of the time it will be even, so instead of making a chart of all values you can make a chart of evens and odds, even + even = even, odd + odd = even, even + odd = odd, odd + even = even. Two for each, so it’s a 50/50 chance.

Daniel Wright
Dec 9, 2018

Each side has 6 possibilities. Broken dawn in conjunction with the question, the meaningful numbers are placed in two categories. Odd and even. Each number that has an odd pairing is put in one category, each number that has an equal pairing is put in another. As both categories have an equal grouping and the answer is the same for two roles. The answer is They are both equally likely.

Deveash Devadas
Dec 7, 2018

When the first die is rolled there is a 50/50 chance that it will be odd or even,( there are three odd and even numbers). On the second roll, there is, again, a 50/50 chance that it will be odd or even. So, the total is equally likely to be odd as even.

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