Aptitude - Probability - Discussion

Discussion Forum : Probability - General Questions (Q.No. 10)
10.
Two dice are tossed. The probability that the total score is a prime number is:
1
6
5
12
1
2
7
9
Answer: Option
Explanation:

Clearly, n(S) = (6 x 6) = 36.

Let E = Event that the sum is a prime number.

Then E = { (1, 1), (1, 2), (1, 4), (1, 6), (2, 1), (2, 3), (2, 5), (3, 2), (3, 4), (4, 1), (4, 3),
      (5, 2), (5, 6), (6, 1), (6, 5) }

n(E) = 15.

P(E) = n(E) = 15 = 5 .
n(S) 36 12

Discussion:
64 comments Page 3 of 7.

Suhail Abdul Rehman Chougule said:   8 years ago
I am sure the answer is 1/6 for this as this event mentioned has something missing.

E = { (1, 1), (1, 2), (1, 4), (1, 6), (2, 1), (2, 3), (2, 5), (3, 2), (3, 4), (4, 1), (4, 3),
(5, 2), (5, 6), (6, 1), (6, 5) }

E(correct) = { (1,1), (1, 1), (1, 2), (1, 4), (1, 6), (2, 1), (2, 3), (2, 5), (3, 2), (3, 4), (4, 1), (4, 3), (5, 2), (5, 6), (6, 1), (6, 5) }
(1,1) will have a probability 2/12 rather than 1/12 un-like other pairs.

If you note (2,1) occurs and (1,2) also occurs and hence (1,1) due to dice A and additionally (1,1) due to dice B and hence the probability of this event is doubled.

Durba said:   7 years ago
The sum of (1, 4) (1, 6) is not prime.

Sirisha said:   7 years ago
1+1=2 here 2 is not a prime number then please explain why (1, 1) used here?

Explain, please.

Williams said:   6 years ago
In the answer, why didn't they count (1,1) twice as it is possible that the number 1 might come on either of the dices, as they calculated (1,2) & (2,1)

Harika said:   6 years ago
Hi when would be the sample space be 12 and when would it be 36? I mean if a dice is thrown twice or if two dice are thrown. What would be the difference in the sample space?

Priyanka said:   6 years ago
Anyone tell me, why not taking (6, 3)?

Sweta said:   6 years ago
Are 4 and 6 prime numbers?

4 and 6 divisible by not only 1 and itself, but it is also divisible by 2, 3.

Priya said:   5 years ago
@All.

Why here we perform addition operation to get a Prime number? Instead of getting total as prime we use (product or subtract).

Why only addition is performed? Can anybody explain this clearly!

Anu said:   5 years ago
@Sowmya.

Yes, the total score here means the sum of numbers.

Trisha said:   5 years ago
@Rutvik.

There (1,1) consider only once because there is no difference between 1, 1 or 1, 1, but there is always a difference between 2, 5 or 5, 2.

Suppose dice 1 and dice 2 have the same no. Like (1, 1) how will you differentiate dice 1 and 2 but if the no. Are change like (1, 2) (2, 1) or (5, 2) (2, 5) you will differentiate dice 1 and 2. That's why.

I hope now you will understand.


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