Question

Mrs. Jacobs drove at a speed of 60 km/h to her mother's house 120 km away. She picked up a box of photos and drove back home at 40 km/h.

Mrs. Jacobs thought to herself, "Well, I drove 60 km/h to get there, and 40 km/h to get home. I must have averaged 50 km/h for the whole trip."

Do you agree with Mrs. Jacob's conclusion? Why or why not?


Can someone please help me with the above problem? Thank you!

Answer 1

I know that you can't generally handle speed like that.

Without trying to go through a long-winded explanation,
let's just calculate her average speed for the whole trip
and see what it was.

Average speed = (distance covered) / (time to cover the distance).

-- On the way going to mother's house . . .

         Distance = 120 km.
         Time       = (120/60) = 2 hours.

-- On the way coming back from mother's house . . .

           Distance = the same 120 km
           Time       = (120/50) = 2.4 hours.

Total distance covered  =  240 km. 
Time to cover the distance = (2 + 2.4) = 4.4 hours.

Average speed  =  (240 km) / (4.4 hours) =  (54 and 6/11) km/hr .

That's why I don't agree with Mrs. Jacobs' conclusion.