probability doubt - where am i wrong?


by jd12345
Tags: doubt, probability
jd12345
jd12345 is offline
#1
Apr27-12, 07:43 AM
P: 260
There is a dice where probability of obtaining a number is proportional to the number itself.
I have done these problems and the probabilties are 1/21, 2/21,/3/21,/4/21,/5/21 and 6/21

In a normal dice when you have to find probabilities of say, 2 and 4 we do it by favourable cases / total number of cases = 2/6 = 1/3

But you cant apply that method in this case - i know that but why?

Probability of 2 or 4 in this case would be 2/21 + 4/21 = 6/21
but why cant we do this by favourable cases / total number of cases?
Phys.Org News Partner Science news on Phys.org
Scientists pinpoint when harmless bacteria became flesh-eating monsters
Asian air pollution affect Pacific Ocean storms
Rocket leak delays space station delivery launch (Update)
tiny-tim
tiny-tim is offline
#2
Apr27-12, 05:31 PM
Sci Advisor
HW Helper
Thanks
tiny-tim's Avatar
P: 26,167
hi jd12345!
Quote Quote by jd12345 View Post
… but why cant we do this by favourable cases / total number of cases?
that formula only applies when each case is equally likely
ssd
ssd is offline
#3
Apr30-12, 01:42 PM
P: 239
Check the three drawbacks of "Classical" definition.


Register to reply

Related Discussions
Simple probability doubt Calculus & Beyond Homework 0
not sure what i did wrong binomial probability Precalculus Mathematics Homework 8
what am I doing wrong? (Probability) Calculus & Beyond Homework 0
How is this probability reasoning wrong?? Precalculus Mathematics Homework 14
Conditional Probability - Teacher says I'm wrong Set Theory, Logic, Probability, Statistics 12