A lift (elevator) has a note saying "maximum 1000 kilograms: maximum 13 persons". Assume that people have weights that are normally distributed with mean 72kg and standard deviation 14kg.
a) Assume that the weights of people in a lift are independent. Calculate the probability that the maximum load is exceeded when 13 people take this lift.
b) Can you think of a reason why in reality the weights of people in a lift might not be independent?