# Thread: Normal variable day-week multiplication?

1. ## Normal variable day-week multiplication?

Hello Statisticians!

I would really appreciate help with this:

The mean number of lemonade glasses sold in a day is 15, standard deviation is 10. Find the probability of selling at least 120 glasses in 7 days (1week)?

The correct answer given in the book is 0.28.

My attempt: 1 - Z<120-(15x7)/(10x7) but I'm getting 0.4.

2. ## Re: Normal variable day-week multiplication?

You probably wanted 1 - P(Z < ...). You need to be careful and frame it as a probability - what you have is just a set.

The other issue you might want to ask yourself is if the standard deviation of the sum of 7 random variables is just 7 times the standard deviation. Hint: it isn't.

3. ## Re: Normal variable day-week multiplication?

Yes I meant P(Z<..), thanks for pointing that out.

Well I figured what I'm doing is wrong, but I can't figure what the right way is.

My question in general is: how do we work out the probability of multiple trials where each follows a normal distribution?

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