Random sampling for quality determination

#1
I have 20 tonnes of chocolate in 800 x 25kg cartons. How much chocolate from how many cartons must I sample to give me x% percent confidence that the sample mean is within y% of the population mean?

I would like to see methodolgy and working too.

I kind of know how to work out how many cartons I need to sample to get a representative sample, but the problem is when I am only taking a proportion of each of the cartons that are part of the sample. If you see what I mean.

Please help someone, this is driving me mental.

Thankyou
 
#2
Hi Marnixkolder,

Welcome to Talk Stats. I am a bit confused. Is 25kg the population mean? I couldn't find any information about the standard deviation. It is easy once we figure out the mean and the sd.