How do you calculate it? A factory manufactures products in two grades of quality. 90% of its revenues comes from manufacturing the first-rate product. How will the revenues change if production of the first-rate product is increased by 10% while the production of second-rate product is decreased by 10%?

$$90\%\cdot r=r_1\implies10\%\cdot r=r_2$$

$$r'=110\%\cdot r_1+90\%\cdot r_2=(110\%\cdot90\%+90\%\cdot10\%)\cdot r$$