The taxi and takeoff time for commercial jets is a random variable x with a mean of 8.5 minutes and a standard deviation of 3.3 minutes. Assume that the distribution of taxi and takeoff times is approximately normal.
c) What is the probability that for 31 jets on a given runway, total taxi and takeoff time will be between 275 and 320 minutes?
help me with this please

Question

The taxi and takeoff time for commercial jets is a random variable x with a mean of 8.5 minutes and a standard deviation of 3.3 minutes. Assume that the distribution of taxi and takeoff times is approximately normal. 
c) What is the probability that for 31 jets on a given runway, total taxi and takeoff time will be between 275 and 320 minutes? 
help me with this please

anonymous · Answer

Standardize, by writing X~N(8.5,3.3^2)
c)P(275<X<320)
Convert X to Z, using $$Z=\frac{ X-\mu }{ \sigma }$$ Can you carry on from here?