Question 353877
Ok so with this you need to do a little algebra to get you answer. 
we are missing the time and distance.
  let just say the distance for both trip is d+d or 2d

Now using the rate problem we will get the time by itself

d = r.t  and t= d/r

      so now we will find time for both trips : t = d/r + d/r
                                                                    t= d/20 +d/30
                                                                      =  3d/60 + 2d/30
                                                                       =  5d/60
                                                                        =   d/12


Now that we have the time lets find the average distance

                                                                                 Average distance = distance / time
                                                                                                             = 2d / d/12
                                                                                                              = 2d x12/d
                                                                                                                =24d/ d
                                         your answer will be 24.