• Oct 2nd 2009, 10:37 AM
sri340
1) A five-digit number is created using the digits 1-5 each once. What is the probability that the number is odd? Express your answer as a common fraction.

2) The sum of the digits of a number is 9. What is the
probability that the number is prime?
• Oct 2nd 2009, 12:28 PM
galactus
#1:

$\frac{5^{4}\cdot 3}{5^{5}}=\frac{3}{5}$

As Plato pointed out, I made a mistake and overlooked your problem said, 'each digit appears once'. Instead of deleting it, let's just say this is the case where the digits repeat.
• Oct 2nd 2009, 12:33 PM
Plato
Quote:

Originally Posted by sri340
1) A five-digit number is created using the digits 1-5 each once. What is the probability that the number is odd? Express your answer as a common fraction.

2) The sum of the digits of a number is 9. What is the
probability that the number is prime?

Quote:

Originally Posted by galactus
#1:
$\frac{5^{4}\cdot 3}{5^{5}}=\frac{3}{5}$

That answer is for numbers with repeated digits.
The problem says "each once".
Try $\frac{4!\cdot 3}{5!}$

#2 hint If the sum of the digits is multiple of three the the number is divisiable by three.
• Oct 2nd 2009, 12:48 PM
galactus
Oh, I overlooked that. (Angry)

Thanks.