In this case we have that the total cost of the tickets is $87.If she is $23, then she is missing:
[tex]87-23 = 64[/tex]
He is missing $64.
If you can save $4 per week we have:
[tex]4x \geq64[/tex]
Where "x" represents the number of weeks to work:
[tex]x \geq \frac {64} {4}\\x \geq16[/tex]
So, Ling must work at least 16 weeks to get the money he needs for concert tickets.
Answer:
16 weeks