Confused about probability solution

In summary, the problem is to find the probability of rolling a sum of 20 with 5 six-sided dice. The solution involves distributing 20 objects into 5 containers, each with a minimum of 1 object. This is equivalent to distributing 15 objects into 5 containers. The solution also considers the condition that each die shows less than or equal to 6, leading to the use of combinations for satisfying this condition. The final step involves subtracting the cases where all 5 dice show a 7 or higher, giving the correct answer of 651/7776. The explanations for the steps (1) and (2) are not clear and require further clarification.
  • #1
davedave
50
0
Here is the problem.

If you roll 5 six-sided dice at the same time, what is the probability that the sum of the roll is 20?

This is the solution that someone did.

The problem can be viewed as distributing 20 identical objects into 5 containers so that each has between 1 and 6 objects inclusive.

Each container must have at least 1 object because each die shows 1 as the smallest number. This reduces to distributing 15 objects.

There are C(15+5-1,15) ways to distribute 15 objects to the 5 containers.

The condition is that each die shows less than or equal to 6.


(1) There are C(5,1)=5 ways to satisfy the condition and there are C(9+5-1,9) combinations.

(2) There are C(5,2)=10 ways to satisfy the condition and there are C(3+5-1,3)
combinations.

So, [C(15+5-1,15)+10*C(3+5-1,3)-5*C(9+5-1,9)]/6^5 = 651/7776

This is the correct answer.


I am very confused about the statements (1) and (2) above.

(a) Why are there C(5,1) and C(5,2) ways to satisfy the condition?

(b) Why are there C(9+5-1,9) and C(3+5-1,3) combinations?

(c) Why do you have to subtract 5*C(9+5-1,9) in the final step of the solution?

Could someone please explain? I would really appreciate your help.
 
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  • #2
Hi davedave! :smile:
davedave said:
The problem can be viewed as distributing 20 identical objects into 5 containers so that each has between 1 and 6 objects inclusive.

No it isn't …

try it with 2 dice adding to 3 …

is that the same as distributing 3 identical objects into 2 containers so that each has between 1 and 6 objects inclusive? :wink:
 

FAQ: Confused about probability solution

What is probability?

Probability is a measure of the likelihood of an event occurring. It is usually expressed as a number between 0 and 1, where 0 represents impossibility and 1 represents certainty.

How do you calculate probability?

The probability of an event occurring is calculated by dividing the number of favorable outcomes by the total number of possible outcomes. This is known as the classical definition of probability.

What is the difference between theoretical and experimental probability?

Theoretical probability is based on mathematical calculations and assumes that all outcomes are equally likely. Experimental probability is based on actual trials or experiments and can vary from the theoretical probability.

What is the difference between independent and dependent events?

Independent events are events where the outcome of one event does not affect the outcome of another event. Dependent events are events where the outcome of one event does depend on the outcome of another event.

How is probability used in real life?

Probability is used in various fields, such as insurance, finance, and weather forecasting, to make predictions and informed decisions. It is also used in everyday situations, such as predicting the likelihood of winning a game or the chances of getting sick.

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