The average life of a motorcycle is 7 years with a standard deviation of 2 years. If the manufacturer is willing to replace only 5% of the motors that fail, for how many years can the company offer a replacement guarantee?

Question

anonymous · Answer

@skullpatrol @rational

rational · Answer

as a start, look at the ztable and find out the "zscore" corresponding to the area of "0.05"

rational · Answer

do you have ztable wid you ?

anonymous · Answer

Yeah

rational · Answer

look up the zscore corresponding to the area 0.05

anonymous · Answer

Okay

rational · Answer

let me know the zscore once you find it

anonymous · Answer

I have no idea, I cant seem to figure this one out

rational · Answer

look at the ztable :

|dw:1432810372760:dw|

whats the zscore corresponding to the area 0.0495 ?

anonymous · Answer

.05

rational · Answer

nope, it is actually "-1.65"

|dw:1432810549294:dw|

anonymous · Answer

-1.6 then

anonymous · Answer

But thats not one of the answers. thats why I dont get it. it says 
2.1 years
2.8 years
3.7 years
4.9 years

rational · Answer

we're not done yet, use zscore formula next

rational · Answer

$$\large -1.65 = \dfrac{x-7}{2}$$

solve $x$

anonymous · Answer

3.7. I feel so stupid now... I dont even know what I was doing wrong. Thank you so much!!

rational · Answer

Looks good! yw