The time spent by a customer at a checkout counter has mean 4 minutes and standard deviation 2 minutes. a) What is the probability that the total time taken by a random sample of 50 customers is less than 180 minutes? b) Find the 30th percentile of the total time taken by 50 customers.

The weight of a radioactive isotope was 96 grams at the start of an experiment. after one hour, the weight of the isotope was half of its initial weight. after two hours, the weight of the isotope was half of its weight the previous hour. if this pattern continues, which of the following graphs represents the weight of the radioactive isotope over time? a. graph w b. graph x c. graph y d. graph z