1) In Question no 7(a),the question clearly says that profit margin should be 10% on sales but in the solution profit margin of 10% is calculated on cost.
2) In question no 7(b), yes it is said that the service provider is unregistered, The aim of the question is to find the tax liability, service tax is payable to the government quarterly but the solution finds the tax liability for the whole year. And if the service provider is unregistered it does not mean that the exemption limit does not exceed 10 lacs in the previous financial year, it may be the default of the assesse to register.