Combinatorics

Hi.

Okay, five digits, two are odd, none can repeat. Let start with the first digit. From 0-9, there are five odd digits (1, 3, 5, 7, 9). The first digit has 5 options.

5 * _ * _ * _ * _

Now let's jump to the fifth digit. If no numbers repeat, there can only be four numbers that digit five can be:

5 * _ * _ * _ * 4

Now fill in the middle digits. Again, no numbers can repeat. Two digits are already filled in, leaving eight for the second digit, seven for the third digit, and six for the fourth.

5 * 8 * 7 * 6 * 4 = 6720

Thanks , got you.. but do you have another approach .. I mean by using the permutation formula??

Abdulla wrote:Thanks , got you.. but do you have another approach .. I mean by using the permutation formula??

IMO, the counting method with a pictorial representation, as done by brb88 is easier.

HT Helps