+0  
 
+1
110
5
avatar+463 

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!

:)

 Apr 30, 2020
 #1
avatar+657 
+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

 Apr 30, 2020
 #2
avatar
+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.

 Apr 30, 2020
 #3
avatar+111329 
+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

 

 

 

cool cool cool

 Apr 30, 2020
 #4
avatar+657 
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
 #5
avatar+463 
+1

yay that's right! thanks CPhill! :)

lokiisnotdead  Apr 30, 2020

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