In this case we have that the total cost of the tickets is $87.If she is $23, then she is missing:
![87-23 = 64](https://img.qammunity.org/2020/formulas/mathematics/middle-school/uhqodt467rrt1j51s8wqhz1yavwqc98zla.png)
He is missing $64.
If you can save $4 per week we have:
![4x \geq64](https://img.qammunity.org/2020/formulas/mathematics/middle-school/9ow6544b8dofxzr32js714kh4tmq5sqycd.png)
Where "x" represents the number of weeks to work:
![x \geq \frac {64} {4}\\x \geq16](https://img.qammunity.org/2020/formulas/mathematics/middle-school/eor78agmudfv18if3mdhwt7a5sptxnjc73.png)
So, Ling must work at least 16 weeks to get the money he needs for concert tickets.
Answer:
16 weeks