Axial loading and the change of wall thickness in a cylinder

Appa

I'm having trouble with understanding how to calculate a wall thickness change in a hollow cylinder unde axial loading. The length, outside diameter and initial wall thickness are given as well as the size of the load, Young's modulus and Poisson's ratio.
I guess this isn't really a hard question but I just can't get my head around it... I computed the shortening of the pipe as well as the change in diameter, but I don't understand how you should go about working out the change in wall thickness, as I guess you need to think about the solid cylinder that's "missing" from inside the hollow one but do I just calculate the amount that the solid one would extend and then subtract it from how much the hollow one has extended? Sounds wrong...