someone handy with probablity help out please

imported_guy

hey guys, 2nd yr mechanical engineering ive been falling back in class a bit and need some help :<

An amplifier consists of 2 independent stages, A and B, connected in series, The output of A is the input to B. The amplifier works if both stages A and B work. If the probablity of A working is 0.72, and the probability of the amplifier working is 0.61 then find the probability that stage B works given that stage A works

and

The lengths of certain machined components are normally distributed with a mean of 1.107m and standard deviation of 0.2m. Find the values that constitute the central 90% of the lengths

thank you very very much

LeixlipRed

The first part is condintional probability. If some event happens how does that effect another event. You should have a formula in your notes? The second question deals with confidence intervals. Again, you should have a formula in your notes. Come back if you have trouble.

imported_guy

LeixlipRed wrote: »

The first part is condintional probability. If some event happens how does that effect another event. You should have a formula in your notes? The second question deals with confidence intervals. Again, you should have a formula in your notes. Come back if you have trouble.

thanks alot looking into it now , edit

would the 1st one be like a glorified deck of cards problem? or rolling 2 dice getting a certain number over 6?

LeixlipRed

Actually the first one is easier than that. A and B are independent.

BQQ

First one seems pretty simple.
If B works 100% of the time the amplifier will work 0.72

imported_guy

i got the first one and i think i figured out the 2nd

z = (x-xbar)/sigma

i think central 90% starts from .2 x .9 = .18 on either side of 1.107

so i need to find the z score between 1.307 and .097

am i doing it the right way?