9.30. The annual-maximum 24-h rainfall amounts on a tropical island have a mean of 320 mm and a standard deviation of 156 mm. The IDF function for the 50-yr storm

on the island is given by i=\frac{6240}{(t+6.87)^{0.65}} \mathrm{~mm} / \mathrm{h} where i is the intensity in mm/h and t is the duration in minutes. Find the annual-maximum 8-h rainfall with a return period of 50000 years.