3D MEAN interval

given a 3d matrix x(16,16,1000) I'd like to sum ou get the mean value in the third dimension given a interval. e.g

given a interval 10 the result would be a matrix x(16,16,100)

given a interval 120 the result would be a matrix x(16,16,50)

and so on

where the third information would be the sum or the mean in the interval.