In Nancy drew the creature of kapu cave on PC how do you the frass weight

Take a look at the clipboard from Dr.Quigly's tent. It shows what the frass looks like, the bug parts, etc. Now, go to the microscope and put a colored bag under it. Then remove everything that is NOT frass. The remaining sediment is the frass. Whatever number is at the top of the microscope, is your frass weight.