Let:
E = Expenses
I = Income
x = number of computers you repair each week
a = average fee that you will charge for each computer you repair
b = cost of replacement parts for each computer
[tex]\begin{gathered} E=bx \\ 500=bx \end{gathered}[/tex]Let's assume you repair 30 computers per week:
[tex]\begin{gathered} 500=bx \\ x=30 \\ 500=b(30) \\ b=\frac{500}{30} \\ b=16.67 \end{gathered}[/tex]--------------------------------------------------
Now, since the incomes must be greater than the expenses:
[tex]\begin{gathered} I=ax \\ I>E \\ so\colon \\ ax>500 \\ \text{let:} \\ x=30 \\ a>\frac{500}{30} \\ a>16.67 \\ a=17 \end{gathered}[/tex]