Let is us suppose club needs to make x T-shirts in order to profit at least $100.
Then the selling price is given by
[tex]\text{Selling Price }= 12x[/tex]
Now, we have been given that it costs $20, plus $6 per T-shirt to make the T-shirts. Thus, the cost price is given by
[tex]\text{Cost Price }= 20+6x[/tex]
Now, profit is given by
Profit = Selling price - cost price
[tex]= 12x-20-6x\\ \\ =6x-20[/tex]
Now, we have been given that profit is at least $100. Therefore, we have the inequality
[tex]6x-20\geq 100[/tex]
Add 20 to both sides
[tex]6x\geq 120\\ \\ x\geq 20[/tex]
Thus, the club needs to sell 20 T-shirts in order to make at least profit of $100