Question

What is the probability of rolling a 20-sided number cube to a number that is prime or a multiple of 3?

Answer 1

0.6 or
`(12)/(20)` or
`(3)/(5)`

Explanation:

Here are the multiples of 3 (that are 20 or below) :

• 3
• 6
• 9
• 12
• 15

• 18

Here are the prime numbers (that are 20 or below):

• 2
• 3
• 5
• 7
• 11
• 13
• 17
• 19

Some numbers are repeated in the two lists.

Here are all the numbers:

• 2
• 3
• 5
• 6
• 7
• 9
• 11
• 12
• 13
• 15
• 18
• 19

12 numbers are prime or a multiple of 3.

As a fraction it will be
`(12)/(20)`, or simplified to
`(3)/(5)`

As a decimal it will be 0.6

I hope it helped!

Answer 2

Probability of rolling a prime or multiple of 3 = 13/20

Probability of rolling a prime = 2/5

Probability of rolling a multiple of 3 = 3/10

Explanation:

let's list primes and multiples of 3 from 1 to 20

Primes:

2 3 5 7 11 13 17 19

Multiples of 3:

3 6 9 12 15 18

Primes OR multiples of 3

2 3 5 6 7 9 11 12 13 15 17 18 19

There's 13 of them, so the probability of rolling a prime or multiple of 3 is 13/20

Probability of rolling a multiple of 3 is 6/20 = 3/10

Probability of rolling a prime is 8/20 = 2/5