You are in the business of printing specialized t-shirts which you can sell for $25 each and at present you are selling 150 t-shirts a month. The cost of a t-shirt is $ 6 and the cost of printing materials for each one is $ 4. The overall cost of running your home-based business for a month, in terms of office expenses, printing machine depreciation, electricity and rent etc. amount to $300.

If your tax rate was 30%, how many t-shirts would you need to sell to make a profit after tax of $ 2,100 per month?