Answer:
The income method calculates the GPD by adding all the income generated by each production stage included in the GDP.
In this case, using the income method:
Farmer sells wheat income = 15
Miller produces flour income = 35 - 15 = 20
Baker makes bread income = 60 - 35 = 25
total GDP 15 + 20 + 25 = 60
If you use the expenditures method, you just account for the price of bread = 60. Both methods should yield the same result.
When using the income method, you must be very careful with depreciation and foreign factors.