Answer: 
5475 cubic ft
Explanation: 
The volume of a cylinder is given by 

where 
r = radius of the base
h = height of the cylinder. 
Now in our case, 
r = 15 / 2 = 7.5 ft 
h = 31ft 
therefore, the volume is 

using π = 3.14 gives 

The right-hand side of the above equation simplifies to give

rounding to the nearest whole number gives 

Hence, the volume of the grain silo is 5475 cubic ft.