The inequality that models the situation is:
[tex]15000 + 4s \geq 450000[/tex]
Solving it, it is found that they need to sell at least 108,750 shirts.
During the day:
Hence, the earnings are:
[tex]T = 15000 + 4s[/tex]
They want to earn at least 450,000, hence:
[tex]T \geq 450000[/tex]
[tex]15000 + 4s \geq 450000[/tex]
Solving it:
[tex]15000 + 4s \geq 450000[/tex]
[tex]4s \geq 435000[/tex]
[tex]s \geq \frac{435000}{4}[/tex]
[tex]s \geq 108750[/tex]
They need to sell at least 108,750 shirts.
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