Mann Inc., had 300,000 shares of common stock issued and outstanding at December 31, 2019. On July 1, 2020, an additional 50,000 shares of common stock were issued for cash. Mann also had unexercised stock options to purchase 40,000 shares of common stock at \(\$ 15\) per share outstanding at the beginning and end of 2020 . The average market price of Mann's common stock was \(\$ 20\) during 2020. What is the number of shares that should be used in computing diluted earnings per share for the year ended December 31, 2020?