My Math Forum  

Go Back   My Math Forum > College Math Forum > Advanced Statistics

Advanced Statistics Advanced Probability and Statistics Math Forum

Thanks Tree1Thanks
  • 1 Post By CRGreathouse
LinkBack Thread Tools Display Modes
September 15th, 2014, 10:35 AM   #1
Senior Member
Joined: Oct 2013
From: New York, USA

Posts: 582
Thanks: 81

Test Probability Without Numbers Question

Every question on a multiple choice test has 5 choices. If there are X questions and the probability of getting every question correct by randomly guessing is Y, what is the probability of getting every question wrong by randomly guessing in terms of X and Y. Is it Y*4^X? A wrong answer is 4 times more likely than a correct answer. Going farther, if the number of questions changes from 5 to Z, does it become Y*(Z-1)^X?
EvanJ is offline  
September 15th, 2014, 12:41 PM   #2
Global Moderator
CRGreathouse's Avatar
Joined: Nov 2006
From: UTC -5

Posts: 16,046
Thanks: 937

Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms
You don't have enough information to decide. Suppose that question 1 has 4 answer choices and question 2 has only 1. Then you chance of guessing all the answers right is 1/4 and your chance of guessing all questions wrong is 0. But if there are 2 choices for each question then the chance of getting all questions right is still 1/4 but the chance of getting all wrong is 1/4 instead of 0.

But if you suppose that each question has the same number of choices, or more generally has the same probability p of being right, then p = Y^(1/X) and so you can compute (1 - p)^X as the probability of getting all questions wrong.
Thanks from EvanJ
CRGreathouse is offline  

  My Math Forum > College Math Forum > Advanced Statistics

numbers, probability, question, test

Thread Tools
Display Modes

Similar Threads
Thread Thread Starter Forum Replies Last Post
Machine of test of prime numbers mente oscura Number Theory 48 May 4th, 2015 12:12 AM
Consecutive Numbers On A Die Probability Question EvanJ Advanced Statistics 7 March 8th, 2014 09:33 PM
Is this a new way to test prime numbers? miket Number Theory 2 April 19th, 2013 12:29 AM
probability question from my test today gbux Probability and Statistics 9 February 25th, 2012 08:34 PM
First Deriv test, finding critical numbers of this.Need help steelcurtain11 Calculus 2 November 1st, 2011 07:51 PM

Copyright © 2018 My Math Forum. All rights reserved.