Year 12s: what will be the first way you research your future options? >>

desperate help on s1!! help greatly needed!!

hello
i need help on s1, it involves simultaneous equations and im finding it hard. please help!! thank you!!

a company produces electronic componenets which have life spans that are normally distributed. only 1% of the components have a life span less than 3500 hours and 2.5% have a life span greater than 5500 hours.

Determine themean and standard deviation of the life spans of the components.

Reply 1

Kernel

When p=1%, z=-2.3263; when p=2.5% z=1.96 (the z values come from the second normal distribution table).
So:
-2.3263=(3500-m)/s rearrange to get m=3500+2.3263s
1.96=(5500-m)/s rearrange to get m=5500-1.96s
5500-1.96s=3500+2.3263s rearrange to get s=2000/4.2863=466.6
m=3500+(2.3263*466.6)=4585.5

Reply 2

ash213

thank u!