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): https://i.imgur.com/HbBEfnH.png 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 :o So... I don't know how to find it (maybe i miss something). Thanks! 
$\Phi^{1}(0.025) = 1.960$ $\sigma = 4.86365$ $\dfrac{w  96}{ 4.86365} = 1.960$ $w = 86.467~g$ 
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I just didn't understand why exponentiation of minus 1. 
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It's the same old table lookup you're used to doing. 
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Now I understood, thanks for help! Have a good week :) 
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it's the symbology for an inverse function 
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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? 
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If the question was about less than 100 grams, then the answer was only: Φ((100−96)/4.86365) Right? 
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