A statistical analysis of 1,000 long-distance phone calls made from a mobile phone indicates that the length of these calls is normally distributed. It is known that half of these phone calls are less than or equal to 240 seconds duration and the standard deviation is 40 seconds.
a) What percentage of these calls lasted less than 180 seconds?
b) How many calls had a length between 180 and 300 seconds?
c) What is the length of a particular call if only 1% of all calls are shorter?
I'm confused by the bit that says ".... known that half of these phone calls are less than or equal to 240 seconds duration and the standard deviation is 40 seconds. ....".
Half the calls are 240 seconds, so is the mean 240? And, how can the SDeviation be used to solve a, b, c?