No matter if you are from the commerce stream or science stream I will make this subject very easy for you. Let me give you a glance at how I teach. If I ask you what is the purchasing value of your mobile. Le t's suppose it is for 30,000. Now, I ask you how much you will give it to me after using it for a year. You might reply 25,000. What is this difference of 5,000 called? Depreciation. So here comes your first formula of national income i.e. Gross value-Depreciation= Net value. + more - less