Three boys with mass 28 35 and 40 kg build a raft joining

Question

Question: Three boys with mass 28, 35 and 40 Kg build a raft joining logs of 20 cm diameter and 2m of length each, in order to use it.

Part A: How many logs must unite to float so that the logs be submerged completely and flush with water?

I need help to solve the above question and consider that the wood density is 788 kg/m³.