The annual-maximum 24-h rainfall amounts on a tropic island have a mean of 320 mm and a standard deviation of 156 mm. the idf curve for the 50-year storm on the island is given by 0.65 6240 ( 6.87) i t   where i is the rainfall intensity in mm/h, and t is rainfall duration in minutes. assume the annualmaximum 24-h rainfall amounts follows the generalized extreme value (gev) distribution. calculate the frequency factor, km, for a return period of 75,000 years