Ok, this is for an into to architectural structures class.

If a cubic foot of concrete weighs 150lbs/ft^3 (p/cf), how many cubes would you need to stack on top of each other for there to be a stress of 3000lbs/in^2 (p/si) on the bottom cube

I know you have to convert. Clearly you dont convert p/cf into p/si...but I really have no idea.

Any help would be greatly appreciated (:

thanks