What proportion

Mahonroy · September 4th, 2009, 07:48 AM

Hello, this question got me a bit confused so thought I would ask:
Each day, a weather forecaster predicts whether or not it will rain. For 80% of rainy days, she correctly predicts that it will rain. For 90% of non-rainy days, she correctly predicts that it will not rain. Suppose that 10% of days are rainy and 90% are non-rainy.
a. What proportion of the forecast are correct?
b. Another forecaster always predicts that there will be no rain. What proportion of these forecasts are correct?

CRGreathouse · September 4th, 2009, 09:36 AM

OK, what are you confused about?

Suppose there are 100 days and it rains on 10 of them. What is 80% of 10? What is 90% of 90? Add these together and you get the number of days out of 100 that the forecaster was correct; this is of course the same as her percentage.

The other forecaster is right for 0% of 10 days and 100% of 90 days. What';s the sum (and thus percentage) there?

Mahonroy · September 8th, 2009, 08:39 AM

Ok that explained it good, thanks!

September 4th, 2009, 07:48 AM	#1
Mahonroy Newbie Joined: Sep 2009 Posts: 15 Thanks: 0	What proportion Hello, this question got me a bit confused so thought I would ask: Each day, a weather forecaster predicts whether or not it will rain. For 80% of rainy days, she correctly predicts that it will rain. For 90% of non-rainy days, she correctly predicts that it will not rain. Suppose that 10% of days are rainy and 90% are non-rainy. a. What proportion of the forecast are correct? b. Another forecaster always predicts that there will be no rain. What proportion of these forecasts are correct?
$Mahonroy is offline$

September 4th, 2009, 09:36 AM	#2
CRGreathouse Global Moderator Joined: Nov 2006 From: UTC -5 Posts: 16,046 Thanks: 933 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms	Re: What proportion OK, what are you confused about? Suppose there are 100 days and it rains on 10 of them. What is 80% of 10? What is 90% of 90? Add these together and you get the number of days out of 100 that the forecaster was correct; this is of course the same as her percentage. The other forecaster is right for 0% of 10 days and 100% of 90 days. What';s the sum (and thus percentage) there?
$CRGreathouse is offline$

September 8th, 2009, 08:39 AM	#3
Mahonroy Newbie Joined: Sep 2009 Posts: 15 Thanks: 0	Re: What proportion Ok that explained it good, thanks!
$Mahonroy is offline$

Thread Tools
$Show Printable Version$ Show Printable Version $Email this Page$ Email this Page
Display Modes
$Linear Mode$ Linear Mode $Hybrid Mode$ Switch to Hybrid Mode $Threaded Mode$ Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Continued-Proportion	Aman Verma	Algebra	3	January 7th, 2014 04:54 PM
Ratio and Proportion I	sachinrajsharma	Algebra	1	March 18th, 2013 09:35 PM
continued proportion	empiricus	Algebra	8	September 20th, 2011 11:40 AM
Direct proportion	MathematicallyObtuse	Algebra	4	January 15th, 2011 08:26 PM
Proportion	haftakhan	Algebra	2	July 5th, 2010 01:49 PM

each day a weather forecast predicts whether or not it will rain,for 80% of rainy days she correctly predicts that it would rain. for 90% of non rainy days she correctly predicts that it will not rain. suppose that 10% of the days are rainy, and 90% of th

Maths.. percentage of non raining days