A truck driver travels a certain distance at 60 miles per hour and returns over the same road at 40 miles per hour. What is her average rate for the round trip?
I know it is a d=rt but I don't know where to begin. Can Please help by explaining thoroughly.
Notice that you are NOT given the distance, but the distance will not
matter, because the answer is independent of the distance. However
we have to let distance be a variable d anyway.
Make this chart:
Distance Rate Time
Going
Returning
---------------------------------------------
Totals XXXX
Fill in the distances, which are both d
Distance Rate Time
Going d
Returning d
----------------------------------------------
Totals XXXX
Fill in their total 2d
Distance Rate Time
Going d
Returning d
----------------------------------------------
Totals 2d XXXX
Fill in the rates, which are 60 and 40
Distance Rate Time
Going d 60
Returning d 40
----------------------------------------------
Totals 2d XXXX
Use the formula 
to fill
in the times:
Distance Rate Time
Going d 60 d/60
Returning d 40 d/40
----------------------------------------------
Totals 2d XXXX
Total up the times
Distance Rate Time
Going d 60 d/60
Returning d 40 d/40
----------------------------------------------
Totals 2d XXXX d/60 + d/40
Now to calculate the average rate
Average rate = 
To simplify that complex fraction, multiply top and bottom by 120:
See how the d's canceled? That shows it didn't matter what the
distance d was, and there is no way of finding out because it
could be any number of miles.
miles per hour.
Edwin
