How many distinct four-digit numbers contain the digits 1, 2, 3, and 4 without repetition, if the digit 2 is always immediately followed by the digit 3?
1 Answer
Sep 2, 2016
Explanation:
Instead of individual digits, think of strings:
#a = # "#1# "
#b = # "#23# "
#c = # "#4# "
There are
Each such arrangement corresponds to a string of
For example:
#bac <=> # "#2314# "
Conversely, if the conditions of the question are satisfied, then a valid sequence of