 Intro to Combinations and Permutations
 Permutations with Repetition
 Permutations without Repetition
 Permutations Formula

License Plates  Android Lock Screen
 Tigers and Bears  Oh My

Combination without Repetition  Combination Formula
 Combining Combinations

The String of Beads  Anagrams
 Combinations with Repetition I
 Combinations with Repetition II
Android Lock Screen
Collapse Content
Show Content
The Android FingerLock screen consists of 9 dots on a square. A code on it consists of 4 to 9 dots visited in a specific order. Assume you can always go from one dot to any nonvisited dot. (In reality some motions are not possible, but you can ignore that for this problem.)
Challenge
How many codes are possible on the Android lock screen?
Please sign in or sign up to submit answers.
Alternatively, you can try out Learneroo before signing up.
Comments
Jacopo Duse Masin
Aug 21, 5:24 AMI don't understand this one:
it shouldn't be 9!, because the first time you can choose among 9, but the second input is a choise among 8, because you can't reselect the one you are on
should't it be
(9*8*8*8)+(9*8*8*8*8)+(9*8*8*8*8*8)+(9*8*8*8*8*8*8)+(9*8*8*8*8*8*8*8)+(9*8*8*8*8*8*8*8*8) ?
Learneroo
Sep 24, 4:34 PM@Jacopo, 9! would give you the number of 9dot permutations, but you also want the total number of 4,5,6,7, and 8 dot permutations. After picking 2 dots, you no longer have 8 dots left...