silo = cylinder
cylinders volume formula is -> V= (pi)(r^2)(h)
to find the radius you just divide the diameter in 2 since half of the diameter = the radius. so our radius would be 9
plug in your numbers and you get to =
v = (pi)(9^2)(40)
do the math and you will get
10178.7601976309
or in terms of pi, 3240pi
sorry if the work is wrong i did this unit in school last year