Junjian opened a 100-share long position in Blackrock stock using the Robinhood iPhone app when Blackrockhad a bid price of $390 and an offer price of $400. A few minutes later he added another 100-share long position in Blackrock stock when the bid price was $440 and the offer price was $450.He sold all 200 shares at the end of the day when Blackrock stock had a bid price of $475 and an offer price of $485.The Robinhood iPhone app features zero trading commissions.

Kayla was also trading Blackrock stock on the same day, using the Interactive Brokers platform that was used in the Math 133A trading competition; her first 100 shares were bought when Blackrock had a bid price of $394.99 and an offer price of $395.00, and her second 100-shares were bought when Blackrock stock had a bid price of $444.99 and an offer price of $445.00. She sold all 200 shares at the end of the day when Blackrock stock had a bid price of $480.00 and an offer price of $480.01.Interactive Brokers charges a $0.005 per share commission.

Who earned a higher net trading profit, Junjian or Kayla?

Make sure you carefully account for trading profits and costs.