You can calculate this percentage by setting up a proportion like this. Note that percent is the amount per 100:
[tex] \frac{3}{40} = \frac{x}{100} [/tex]
Use cross multiplication:
[tex]\frac{3}{40} = \frac{x}{100} \\ \\ 3*100 = 40*x \\ \\ 300 = 40x \\ \\ \frac{300}{40} =x \\ \\ x = 7.5[/tex]
The brain represents 7.5% of an average person's weight.
