First find the total number of pine cones: 8 trees * 200 pine cones/tree = <<8*200=1600>>1600 pine cones
Then multiply that number by 30% to find how many fall on Alan's roof: 1600 pine cones * 30% = <<1600*30*.01=480>>480 pine cones
Then multiply that number by the weight of each pine cone to find the total weight of the pine cones: 480 pine cones * 4 ounces/pine cone = <<480*4=1920>>1920 ounces
#### 1920