Questions about the world of GRE Math from other sources and general math related questions.
sanjana.ahsan
Students
 
Posts: 1
Joined: Thu Dec 19, 2013 6:09 pm
 

probability of independent events

by sanjana.ahsan Thu Jun 05, 2014 10:33 pm

This is a question in my 5lb book of practice questions:

The probability of rain in Greg's town on Tuesday is 0.3. The probability that Greg's teacher will give him a pop quiz on Tuesday is 0.2. The events occur independently of each other.

what is the probability that either or both events occur?

the answer is 0.3 + 0.2 - (0.3)(0.2) = 0.44 but I don't understand why the probability of both events occurring is subtracted because the question asks for the occurrence of both evens to be included..
tommywallach
Manhattan Prep Staff
 
Posts: 1917
Joined: Thu Mar 31, 2011 11:18 am
 

Re: probability of independent events

by tommywallach Fri Jun 06, 2014 5:31 pm

Hey Sanjana,

This question was already answered on this thread: 5lb-ch23-q44-t10192.html

Enjoy!

-t