There are 60 ridges on a vinyl record. Jerry has 4 cases, each with 3 shelves that can hold 20 records each. If his shelves are 60% full, how many ridges are there on all his records?
First find the total number of records that can fit on all the shelves: 4 cases * 3 shelves/case * 20 records/shelf = <<4*3*20=240>>240 records
Then multiply that number by 60% to find the number of records actually on the shelves: 240 records * 60% = <<240*60*.01=144>>144 records
Then multiply that number by 60 to find the number of ridges on all the records: 144 records * 60 ridges/record = <<144*60=8640>>8640 ridges
#### 8640