The weight of human btain is 15N the buoyant force supplied by fluid around brain about 4.5N does this mean that there must be at least 14.5N surrounding brain?

The answer is no. The weight of the human brain is 15 N, and the buoyant force supplied by the fluid around the brain is 4.5 N. This means that the net force on the brain is 10.5 N downward. However, this does not mean that there must be at least 14.5 N of fluid surrounding the brain. The fluid could be less dense than water, in which case it would exert a smaller buoyant force. Additionally, the brain could be floating in a cavity that is not completely filled with fluid. In this case, the buoyant force would be less than the weight of the brain even if the density of the fluid were the same as that of water.