I am struggling to see where the 1/3 comes from in front of the equation below, could anyone tell me why it is there please. The only correlation I can make is becuase there are 3 terms, is this correct?

Iv = 2(12 x 113.4^3) / 12 + (400 x 36.6^3) / 12 - (376 x 24.6^3) / 12

Iv = 1/3 ( 35000000 + 19610000 - 5597000)

I know how the figures are calculated but don't know why the 1/3 comes from and why.

Thanks in advance