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

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

Now, profit is given by
Profit = Selling price - cost price

Now, we have been given that profit is at least $100. Therefore, we have the inequality

Add 20 to both sides

Thus, the club needs to sell 20 T-shirts in order to make at least profit of $100