averaging of image sequences

hi,
i am trying for element to element averaging of image sequences.

an image 'img' of size (m,n,10) for example
i need something like this:
mn(:,:,1) = img(:,:,1);
mn(:,:,2) = 0.5*(img(:,:,1)+img(:,:,2));
.
.
.
mn(:,:,10) = 0.1*(img(:,:,1)+...+img(:,:,10));

thanks