Expression help with time remapping

Hi

I have a 25fps Precomp with an animation that changes on every frame.

In the master comp I want to apply an expression to its time remap property so that it holds on each consecutive frame for a given duration, eg on Frame 1-15 we see frame 1 of precomp, on frame 16-30 we see frame 2, and so on.  I can't figure this out. Tried posterizeTime and some other bits to no avail and can't find it elsewhere on the web.

Thanks, Steve