You want to build a raft that can hold you (80 kg) plus some supplies (40 kg) for a long...

You want to build a raft that can hold you (80 kg) plus some supplies (40 kg) for a long trip. You decide to make the raft out of logs that are each 20 cm in diameter and 3.0 m long. If the density of each log is how many logs do you need?