Consider that the revenue is calculated as,
![\text{Revenue}=\text{Price per student}*\text{No. of student}](https://img.qammunity.org/2023/formulas/mathematics/college/eknwxh4lrvez7rh2ztay8n12f2w24rbtw0.png)
The current revenue of Mrs. Nicely is,
![R_(\circ)=20*21=420](https://img.qammunity.org/2023/formulas/mathematics/college/1yvpjpe6skxudrnqd2it3fh8nlwd5buhjs.png)
Let Mrs. Nicely lost 'x' students due to the fee hike. Now, there are (21-x) students remaining in her.
Given that every $4 increase in price results in loss of 1 student. The initial price was $20. So the price after 'x' students left will be (20+4x)$.
So the revenue function becomes,
![R(x)=(20+4x)(21-x)=-4x^2+64x+420](https://img.qammunity.org/2023/formulas/mathematics/college/nuv7w9bi8exnqhl7si9ayt62p00l4zoet3.png)
Now, obtain the stationary points as,
![\begin{gathered} R^(\prime)(x)=0 \\ -8x+64=0 \\ 8x=64 \\ x=8 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/o6td66iz1nts7ytn60zt51uniumvxgqn89.png)
Apply the second derivative check,
![\begin{gathered} R^(\doubleprime)(x)=(d \square)/(dx)(R^(\prime)(x)) \\ R^(\doubleprime)(x)=(d\square)/(dx)(-8x+64) \\ R^(\doubleprime)(x)=-8 \\ R^(\doubleprime)(x)<0 \\ R^(\doubleprime)(8)<0 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/9qx1lwtqpodo3b7j93oj2ctagnqfl1dn2c.png)
Since the second derivative is less than zero, the revenue function is maximum at x=8.
Thus, Mrs Nicely will have maximum revenue when 8 students have left, and the remaining students are (21-8=)13.
The corresponding price per student is calculated as,
![P=20+4x=20+4(8)=20+32=52](https://img.qammunity.org/2023/formulas/mathematics/college/rzj1lw6l0hyscs1acs0sd8o8ejhqw2f688.png)
So Mrs. Nicely should charge $52 for tutoring in order to maximize her revenue.
And the maximum revenue is calculated as,
![R_(\max )(x)=R(8)=52*13=676](https://img.qammunity.org/2023/formulas/mathematics/college/sux55ey8ofg7557aw4kthvbt8g5nay9z6k.png)
Thus, the maximum revenue she can expect is $676 each week.