# Thread: Standard deviation and mean?

1. ## Standard deviation and mean?

The duration of direct flights from London to Singapore in a particular year followed a normal
distribution with mean μ and standard deviation σ .
92 % of flights took under 13 hours, while only 12 % of flights took under 12 hours 35 minutes.
Find μ and σ to the nearest minute.

I'm stuck here, i put in the formula and then how do i find inverse normal? Step by step instructions? I have a TI84 if that helps.
Thanks

2. ## Re: Standard deviation and mean?

Originally Posted by Koko1211
The duration of direct flights from London to Singapore in a particular year followed a normal
distribution with mean μ and standard deviation σ .
92 % of flights took under 13 hours, while only 12 % of flights took under 12 hours 35 minutes.
Find μ and σ to the nearest minute.

I'm stuck here, i put in the formula and then how do i find inverse normal? Step by step instructions? I have a TI84 if that helps.
Thanks
Usually you look it up in a table.

For a standard normal

92% corresponds to a z-score of 1.405

12% corresponds to a z-score of -1.175

That should be enough to finish the problem.