Question

How many five-digit numbers are there that contain the digit 1 (at least) three times in a row?

Answer 1

Answer:

So you say you want some 5 digit numbers eh? Let’s consider some boundaries. 5 digit numbers range from 10,000 - 99,999. So our number is guaranteed to be between 0 and 90000.

Sometimes it’s much easier to work with concrete things. Such as the number of numbers that don’t contain any 3s. It’s really more difficult to put our finger on “at least 1” anything so we try to steer clear of it.

Anyways we’ve got 90000 numbers that have at least 1 or 0 3’s. Find one and we know the other.

So no 3’s at all right? Well you have 8 choices for the first digit (can’t choose 0 and you can’t choose 3). And then for the remaining 4 digits you have 9 choices. So that’s

8∗94=52488  

Back to our initial equation, we have

90000−8∗94=37512  5 digits numbers without any 3s.

A quick (or maybe not so quick) matlab script confirms such calculations.

credits to : Trevor Squires