# Number Of Numbers

Discrete Mathematics Level pending

A positive integer N has the decimal representation

$$N={ d }_{ 1 }{ d }_{ 2 }{ d }_{ 3 }{ d }_{ 4 }......{ d }_{ n }$$

such that ,

$$0\quad <\quad { d }_{ 1 }\quad \le \quad { d }_{ 2 }\quad \le \quad { d }_{ 3 }\quad .........\le \quad { d }_{ n }$$

$$d_k$$ here represent a digit at $$(n-k)^{th}$$ place.

Find the total number of numbers of this type which are less than $${ 10 }^{ 9 }$$

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