First find the total weight of the silverware per setting: 4 ounces/piece * 3 pieces/setting = <<4*3=12>>12 ounces/setting
Then find the total weight of the plates per setting: 12 ounces/plate * 2 plates/setting = <<12*2=24>>24 ounces/setting
Then add those two amounts to find the total weight per setting: 12 ounces/setting + 24 ounces/setting = <<12+24=36>>36 ounces/setting
Then multiply the number of tables by the number of settings per table to find the number of table settings: 15 tables * 8 settings/table = <<15*8=120>>120 settings
Then add the 20 backup settings to find the total number of settings: 120 settings + 20 settings = <<120+20=140>>140 settings
Then multiply the total number of settings by the weight per setting to find the total weight: 140 settings * 36 ounces/setting = <<140*36=5040>>5040 ounces
#### 5040