My Math Forum  

Go Back   My Math Forum > High School Math Forum > Probability and Statistics

Probability and Statistics Basic Probability and Statistics Math Forum

LinkBack Thread Tools Display Modes
October 1st, 2017, 06:13 PM   #1
Joined: Oct 2017
From: USA

Posts: 1
Thanks: 0

Probability question

Hi, hopefully I can explain this well. So let's say you roll for a chance to get a number from 1-21. Every time you get 21, you roll for a chance to get 1-18. You do this until you get every number in 1-18 (1,2,3,4,5,6,7, etc) once. How many 1-21 rolls would this take? And can you provide a formula of some sort? Thanks!
marinahaim is offline  
October 3rd, 2017, 06:06 PM   #2
Senior Member
Joined: Oct 2013
From: New York, USA

Posts: 561
Thanks: 79

The formula for how many times it will take to get every number from 1 to 18 is the sum of 18/x for the 18 values of x. To the nearest whole number, the expected numbers of times this will take is 63. The expected rolls of the 1 to 21 where only 21 is a success is 63*21 = 1,323. I'm not guaranteeing that I understand the question, and it's important to know that the probabilities you're asking about do not have guarantees. For example, you could hypothetically roll a 10 for 100 consecutive times. A question like "how many coins does it take to guarantee at least 2 heads" may sound simple, but it doesn't have an answer.
EvanJ is offline  

  My Math Forum > High School Math Forum > Probability and Statistics

probability, question

Thread Tools
Display Modes

Similar Threads
Thread Thread Starter Forum Replies Last Post
probability question amcgl064 Probability and Statistics 7 September 28th, 2012 07:20 AM
probability question gbux Probability and Statistics 3 February 15th, 2012 06:07 PM
Probability question henryzz Advanced Statistics 1 November 23rd, 2010 11:24 AM
Probability Question herohenson Probability and Statistics 2 January 25th, 2010 07:41 PM
Probability Question skiing64 Algebra 2 September 3rd, 2009 05:54 PM

Copyright © 2017 My Math Forum. All rights reserved.