A raft is able to float on water in spite of the fact that the material it may be made of is heavier than water. This is due to a buoyant force acting upwards created by the water that the raft displaces. The raft is pushed up by a force equal to the weight of water displaced by it.
Here, a raft with mass 13 kg and total volume 100 m^3 is floating on water. The density of water is 1000 kg/m^3 and the raft can displace a maximum 100 m^3 or 100*10^3 kg of water. If a mass M is placed in the raft, it will continue to float until M + 13 = 100*10^3 kg.
The raft can carry a mass 100*10^3 - 13 or approximately 100 tons while staying afloat.