I have a problem like this:
In a car factory a study was made to calculate how long each worker takes to build a part. The mean μ = 75 seconds and the standard deviation σ = 6 seconds.
Find an interval in which the probability of a worker building a part in t seconds, t belonging to that interval, is 80%.
I can calculate this using my calculator (I think that is the point of that problem), but I would like to know if it is possible to do this manually, if there is some formula or anything for this and if yes, how.
Thanks