-2
$\begingroup$

How do I go about solving this question on probability?

Two dice are thrown 5 times. Calculate the probability to obtain the same number on both dice 2 times.

One thing i know that to obtain any number on one die is 1/6. Need help in moving ahead.

$\endgroup$
  • 1
    $\begingroup$ Can you solve: "Two dice are thrown 1 time. Calculate the probability to obtain the same number on both dice"? $\endgroup$ – Did Apr 16 '17 at 9:08
  • 1
    $\begingroup$ Then, can you solve: "The same experiment, with probability of success p, is performed 5 times. Calculate the probability that it is successful 2 times"? $\endgroup$ – Did Apr 16 '17 at 9:10
2
$\begingroup$

The possible rolls of two dice are: \begin{matrix} 11 & 12 & 13 & 14 & 15 & 16 \\ 21 & 22 & 23 & 24 & 25 & 26 \\ 31 & 32 & 33 & 34 & 35 & 36\\ 41 & 42 & 43 & 44 & 45 & 46\\ 51 & 52 & 53 & 54 & 55 & 56\\ 61 & 62 & 63 & 64 & 65 & 66\\ \end{matrix}

$\frac 6 {36}$ of these rolls have the dice the same number, $\{11,22,33,44,55,66\}$, and $\frac {30} {36}$ don't have the same number.

Now think about tossing two dice 5 times. How can you get matching dice exactly two of the five tosses? First and second toss of the two dice or first and third toss or...or fourth and fifth toss. There are $\binom 5 2=10$ ways to choose two of five tosses. The rest of the tosses (three of them) will have non-matching dice. Each of the two tosses with matching dice will give a probability of $\frac 6 {36}$. Each of the three tosses with different dice will give $\frac {30} {36}$.

We end up with our probability. $\binom 5 2(\frac 6 {36})^2(\frac {30} {36})^3$

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.