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# A level Maths Watch

1. I need help on this question on S1:

Jam is sold in jars and the mean weight of the contents is 108 grams. Only 3% of jars have contents weighing less than 100 grams. Assuming that the weight of jam in a jar is normally distributed find:

a) the standard deviation of the weight of jam in a jar

b) the proportion of jars where the contents weigh more than 115 grams.
2. (Original post by chromesilver7)
I need help on this question on S1:

Jam is sold in jars and the mean weight of the contents is 108 grams. Only 3% of jars have contents weighing less than 100 grams. Assuming that the weight of jam in a jar is normally distributed find:

a) the standard deviation of the weight of jam in a jar

b) the proportion of jars where the contents weigh more than 115 grams.
Part a will be related to the amount of data within a certain number of standard deviations from the mean (I can't remember the values right now, but x% of data is within 2 standard deviations). So then 100 is 8 away from the mean, which is however many standard deviations, then 8/number of standard deviations = the standard deviation (I hope that made sense, sorry!)

Part b will simply be an ordinary normal distribution question using the standard deviation as found in part a
3. Yes, that does make sense for part (a)... However, I got stuck while doing part (a). A solution bank showed the changing of 0.03 into 1.8 something... How did they get to that???
4. (Original post by chromesilver7)
Yes, that does make sense for part (a)... However, I got stuck while doing part (a). A solution bank showed the changing of 0.03 into 1.8 something... How did they get to that???
Scratch what I said for part a, I think you're supposed to use the formula for finding z! z for .97 (1-.03) (from reverse normal tables, this will be your -1.8 something)=100-108/the standard deviation. Rearrange and solve!
5. Oh okay, I get it now, thanks so much!
6. hey guys
Im currently doing a c3 question and seem to be struggling with a question.
If anyone could help me out that would be great.It would much appreciated if you could show me the steps as well
Thanks

The question is
find the inverse for the function f(x)=3lnx-ln3x
7. (Original post by Alevel4)
hey guys
Im currently doing a c3 question and seem to be struggling with a question.
If anyone could help me out that would be great.It would much appreciated if you could show me the steps as well
Thanks

The question is
find the inverse for the function f(x)=3lnx-ln3x
The way I've always found an inverse is to let the function = y then make x the subject. Then your f(y)=inverse function and you can just replace the ys with xs. I won't show you every step, you've got to try it yourself.
8. yeah thats how i do but
i got e^y=x^3-3x
dont know how to proceed from there
if you could help it would be great

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