Richard can mow the lawn three times as fast as Terri. Together, they finish mowing the lawn in 20 minutes. Which of the following equations would describe how many minutes it would take Richard to mow the lawn by himself? a. 1/x+1/3x=1/20 b. 1/x+1/3=1/20 c. 20 (1/x+1/3x)=1/20 d. x+3x=1