# Thread: Further Maths AS - S2 stuck on question!

1. ## Further Maths AS - S2 stuck on question!

Ok maybe I'm not thinking clearly enough but here's the question with a stupid story to make it sound realistic:

Q) The fluorescent light tubes made by the company "Well-lit" have lifetimes which are normally distributed with mean 2010 hours and standard deviation 20 hours. The company decides to promote its sales of the tubes by guaranteeing a minimum life of the tubes, replacing free of charge any tubes that fail to meet this minimum life.

If the company wishes to have to replace free only 3% of the tubes sold, find the guaranteed minimum it must set.

I've tried to make sense of the wording but I'm too slow to do that!

So I know that $\mu = 2010, \sigma = 20$

I think I need to do something with the 3% in order to get a number value which I can put into this equation:

$\frac{x - \mu}{\sigma}$

I'm just really confused with all the information; we've learnt how to do questions in stats but now they're in real life situations and it can get a tad confusing.

I know I'm not as good as everyone here but I just need some help with this question

2. Originally Posted by Femto
Ok maybe I'm not thinking clearly enough but here's the question with a stupid story to make it sound realistic:

Q) The fluorescent light tubes made by the company "Well-lit" have lifetimes which are normally distributed with mean 2010 hours and standard deviation 20 hours. The company decides to promote its sales of the tubes by guaranteeing a minimum life of the tubes, replacing free of charge any tubes that fail to meet this minimum life.

If the company wishes to have to replace free only 3% of the tubes sold, find the guaranteed minimum it must set.

I've tried to make sense of the wording but I'm too slow to do that!

So I know that $\mu = 2010, \sigma = 20$

I think I need to do something with the 3% in order to get a number value which I can put into this equation:

$\frac{x - \mu}{\sigma}$

I'm just really confused with all the information; we've learnt how to do questions in stats but now they're in real life situations and it can get a tad confusing.

I know I'm not as good as everyone here but I just need some help with this question
If only 3% of the bulbs do not meet the requirement, then solve

P(X<=k)=0.03 where k is the minimum lifetime.

Oh thank you so much; so all I need to do is find the number for 97% and make it negative? Then stick the numbers into that equation $\frac{x - \mu}{\sigma}$? Or not?