Question

@jim_thompson5910 help again please!

in how many ways can nine people be lined up for a photograph if clark and billy must not stand next to each other

Answer 1

Count the number of ways to order the group with clark and billy next to each other. Call this count n


Then count the number of ways to order everyone (regardless of where clark and billy are). Call this count m


Subtract: m - n

Answer 2

http://openstudy.com/study#/updates/512d9d0ae4b02acc415e65ab