3n = 1/81; divide both sides by 3 to isolate n:
3n/3 = 1/81 divided by 3. Remember that when you divide, you use the reciprocal and multiply instead!!
n = 1/81(1/3) = 1/243. Now plug 1/243 back in for n, and 3/243 is equivalent to 1/81
 
        
             
        
        
        
V ≈ 15.63cm^3
volume of a cube = side × side × side = units^3
Volume = 2.5*2.5*2.5 = 15.63^3
<em>Hoped this helped!</em>