Estimate how much the brain would swell due to osmosis if the concentration of the cerebrospinal ±uid fell to 280mm. in other words, ³nd what the new volume

Since the average osmotic pressure of the brain is 300mM and the concentration of the solution is 280mM, that means there is 20mM difference between the brain cells and CSF. This will induce osmosis and cause water to move from the CSF into the brain until the concentration becomes same. Assuming brain volume is 1200ml and the CSF constantly replaced., then the estimation of fluid should be: 20mM/300mM * 1200ml= 80 ml
If it made percent it should be: 80ml / 1200ml= 6.67%

Spherical shaped cell might increase the surface area and decrease the volume.