April 20th, 2018, 08:45 PM  #1 
Member Joined: Mar 2017 From: Israel Posts: 70 Thanks: 2  Statistics
Hello Can you help me please about the next exercise: The weight of a thirdgeneration mobile phone is normal (normal distribution), with an average of 96 grams. It is known that 90% of these phones are between 88 grams and 104 grams. The first question was to find standard deviation of the phones weight, I found it (if I didn't made a mistake): So the standard deviation is 4.863 (from table Z). Now there is another question: What is the weight that only 2.5% of these phones weigh less? I tried to find but i had insane answers So... I don't know how to find it (maybe i miss something). Thanks! 
April 20th, 2018, 09:01 PM  #2 
Senior Member Joined: Sep 2015 From: USA Posts: 2,197 Thanks: 1152 
$\Phi^{1}(0.025) = 1.960$ $\sigma = 4.86365$ $\dfrac{w  96}{ 4.86365} = 1.960$ $w = 86.467~g$ 
April 20th, 2018, 11:53 PM  #3 
Member Joined: Mar 2017 From: Israel Posts: 70 Thanks: 2  
April 21st, 2018, 12:17 AM  #4 
Senior Member Joined: Sep 2015 From: USA Posts: 2,197 Thanks: 1152  
April 21st, 2018, 01:20 AM  #5 
Member Joined: Mar 2017 From: Israel Posts: 70 Thanks: 2  
April 21st, 2018, 06:59 AM  #6 
Member Joined: Mar 2017 From: Israel Posts: 70 Thanks: 2  
April 21st, 2018, 10:17 AM  #7 
Senior Member Joined: Sep 2015 From: USA Posts: 2,197 Thanks: 1152  
April 27th, 2018, 05:30 PM  #8  
Member Joined: Mar 2017 From: Israel Posts: 70 Thanks: 2  Quote:
There is another question in same exercise (which I wrote in the current post), can you tell me please if I solved correctly? The question is: What is the relative frequency of 3G (thirdgeneration) phones weighing more than 100 grams? My answer: Because of normal distribution and there is 90% in 88104 grams, then in 100104 grams there is 22.5%, and above 104 there is 5%, therefore the answer is 22.5%+5%=27.5%=0.275 Right?  
April 27th, 2018, 06:15 PM  #9  
Senior Member Joined: Sep 2015 From: USA Posts: 2,197 Thanks: 1152  Quote:
 
April 28th, 2018, 03:13 AM  #10 
Member Joined: Mar 2017 From: Israel Posts: 70 Thanks: 2  

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