Roberto:  let  t  represent his time

                                  d = rt = 6t

Darrel:  t + 1  represents his time

                                d = 4(t + 1)

When Roberto catches up to Darrel:

              6t  =  4(t + 1)

              6t  =  4t + 4

              2t  =  4

              t =  2

Return