National income is one of those topics that looks confusing only because too many similar terms are taught together. Once the sequence is understood, the topic becomes far more manageable.
An economy produces goods and services. The total value of what is produced within the domestic territory is called GDP. If you add income earned by residents from abroad and subtract income earned inside India by non-residents, you move toward GNP.
Machines wear out. Buildings lose useful life. This loss is depreciation. When depreciation is deducted from gross figures, we move from gross to net figures. That is why NNP is a net measure rather than a gross one.
GDP → GNP → NNP → National Income. Each move happens because of a clear adjustment, not because the textbook wanted more formulas.
Whenever a numerical question appears, first identify whether the base figure is domestic or national and whether it is gross or net. Once that is clear, most mistakes reduce sharply.