studentnow wrote:

With the five numbers 1-5 how many such 5-digit numbers are possible in which even numbers are not adjacent

1.120

2.100

3.96

4.72

5.48

The more I do em, the easier combinatorics get

Ans D.

1,2,3,4,5. Adjacent meaning "next to eachother."

Lets find the total number of ways we can position these 5 numbers. 5!=120.

Now lets find the number of ways that 4 and 2 ARE next to each other.

Make 4 and 2 one unit. 4-2,1,2,5 --> 4!. We aren't done yet. We must multiply 4! by 2 because we have yet to consider 2-4,1,3,5

4!*2=48. 120-48=72.