However 123 is not snakelike (2 is greater than 1 but not 3) and 91772
is not snakelike (7 is greater than 1 but not 7).
a How many 4-digit snakelike numbers contain each of the digits 1, 2,
3, 4 exactly once?
b How many 3-digit snakelike numbers have 6 as their middle digit?
c A 7-digit snakelike number starts with digit 9. Explain why its last
digit cannot be 0.
d A 4-digit snakelike number starts and ends with the same digit. The
positive differences between pairs of adjacent digits are written down
in order to form a 3-digit number. Explain why this 3-digit number
is always snakelike.