He must sell 4 chairs to make a profit.
Explanation
Lets assume, the number of chairs he must sell to make profit is [tex]x[/tex]
He charges 65 dollar for a chair. So the total selling price for [tex]x[/tex] number of chairs [tex]= 65x[/tex] dollar
The materials for a chair cost 45 dollar. So, the total cost of materials for [tex]x[/tex] number of chairs [tex]= 45x[/tex] dollar and there is a 75 dollar fee for a booth at the market.
Now for making profit.....
[tex]65x>45x+75\\ \\ 65x-45x>75\\ \\ 20x>75\\ \\ x> \frac{75}{20}\\ \\ x>3.75[/tex]
So, he must sell 4 chairs to make a profit.
