In R, you can assign your own operators using %[characters]%. A trivial example:
'%p%' <- function(x, y){x^2 + y}
2 %p% 3 # result: 7
While I agree with BlueTrin that %% is pretty standard, I have a suspicion %/% may have something to do with the sort of operator definitions I showed above - perhaps it was easier to implement, and makes sense: %/% means do a special sort of division (integer division)