Robber Co June 2012

Hi John,
In this question we are asked to advise if the company should make in house or buy from outside. I wanted to ask if I show my workings as Cost of producing in house and then compare it with Offered price will it be accepted because I am not so comfortable in doing the incremental cost workings and that goes for all the questions of incremental costs. I am more comfortable in calculating it separately and comparing and then discussing.
Please advise.