One day, River writes down the numbers 1,2,...,999

River chooses one of the digits written down at random. What is the probability that River chooses a 7?

Thank you and please provide an explanation!

:)

lokiisnotdead Apr 30, 2020

#1**+1 **

OK Here is my best shot :D

There are 280 numbers contain the digit 7 between 0 and 999.

**So 280/999**

Here to help

:D

LuckyDucky Apr 30, 2020

#2**+1 **

If repeated digits are counted, such as 77, 777.....etc, then there are** 300 sevens** between 1 and 999.

If repeated digits are NOT counted, then there are** 271 sevens** between 1 and 999.

Then, the probability is either 300 / 999 or 271 / 999.

Guest Apr 30, 2020

#3**+4 **

I get a slightly different result

The number of digits written down =

9 + 2(99 - 10 + 1) + 3(999 - 100 + 1) =

9 + 2 ( 90) + 3(900) =

2889

The number of 7's that are written down =

10*10 + 10(10) + 100 = 300

So.....the probability of selecting a "7" = 300 / 2889 = 100/963

CPhill Apr 30, 2020

#4**0 **

Ahhhh wait CHphill I'm doing an AOPs online course and I remember getting this question right in fact you are right the answer is indeed **100/963**

Great work as usalll!

Here to help

:D

LuckyDucky
Apr 30, 2020