+1671 Find the number of ways of arranging the numbers 1,2 ,3, 4, 5, 6 in a row so that the product of any two adjacent numbers is at least 5.
0 At least 5 means we need 1 in between 5 and 6. imagine this as a block. So including the block there are 4 total numbers. These numbers have \(4!=24\) ways to arrange them. Then the block has 2 permutations. There can be anyother number next to each other as it is always greater than 5 so:
\(24\times2=\boxed{48}\)
+33616 Any permutation starting 1, 5 , ... 4! of them
Any permuttion starting 1, 6, ... 4! of them
Any permutation ending ..., 5, 1 4! of them
Any permutation ending ..., 6, 1 4! of them
Any permutation starting 5, 1, 6, ... 3! of them
Any permutation starting 6, 1, 5, ... 3! of them
etc. ...
