My Math Forum What proportion

 September 4th, 2009, 07:48 AM #1 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?
 September 4th, 2009, 09:36 AM #2 Global Moderator     Joined: Nov 2006 From: UTC -5 Posts: 16,046 Thanks: 938 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?
 September 8th, 2009, 08:39 AM #3 Newbie   Joined: Sep 2009 Posts: 15 Thanks: 0 Re: What proportion Ok that explained it good, thanks!

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### Maths.. percentage of non raining days

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