MathDB

Kelvin the Frog likes numbers whose digits strictly decrease, but numbers that violate this condition in at most one place are good enough. In other words, if

d_i

denotes the

i

th digit, then

d_i\le d_{i+1}

for at most one value of

i

. For example, Kelvin likes the numbers

43210

132

, and

3

, but not the numbers

1337

and

123

. How many

5

-digit numbers does Kelvin like?

Problem Statement