Probability Of Both Primes

Probability Level 2

Let $A$ be a fair die with sides numbered 1 through 20. Let $B$ be a fair die with sides numbered 1 through 11. If $A$ and $B$ are each rolled once, what is the probability that both dice land on a prime number?

\dfrac{5}{11}

\dfrac{2}{11}

\dfrac{10}{11}

\dfrac{2}{5}

1 solution

A Former Brilliant Member
Apr 10, 2016

Number of primes from $1$ to $20$ is $8$ . Therefore probability that we get a prime number on $A$ is $\dfrac 8 {20}=\dfrac 2 5$

Number is primes from $1$ to $11$ is $5$ . Therefore probability that we get a prime number on $B$ is $\dfrac 5 {11}$

Therefore probability that we get a prime number on both $A$ and $B$ is $\dfrac 2 5 \times \dfrac 5 {11}=\dfrac 2 {11}$ .

Probability Of Both Primes

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