The mean time for running a mile is 450 se4conds with a standard deviation of 40 seconds. What time would you need to beat to be in the top 10%?

Answer 1

Assuming normal distribution, the z-score for 90% is 1.288, which means that the time is μ-1.288σ
=450-40*1.288 seconds.

For z-score, see:
http://www.regentsprep.org/Regents/math/algtrig/ATS7/ZChart.htm

Answer 2

6.6413