Rate problems

[ssr174]

I have a question out a solution approach to a word problem including rates.

See chapter 6 of word problem study guide, word problem strategies. Problem set 1.

The question asks for the avg speed of the car for the combined rates/trip.

Why can't you solve this problem with the strategy outlined in chapter 2 Rates and Work. Ie adding the rates together her. ?


Thanks

[aditya8062]

i am not sure which chapter you talking of but i think i get what u r saying
i think u r asking that when we are asked to find the average of ,lets say ,30 and 40 then all we do is we add these two numbers and divide by 2 and we get average as 35 !!
but u cannot do the same stuff when u r asked to find the average speed if u traveled a distance D @30 kmph and traveled again the same distance @ 40 kmph .
well the reason is the average speed is calculated as the ratio of total distance to total time .
to appreciate this concept u must understand that the speed formula is distance /time .so indeed the formula for average speed is total distance /total time

[RonPurewal]

hi ssr174,

this thread is now locked, because 2 things are wrong:
1/
this is the wrong folder. if this question is from a MGMAT book, you should post it in the manhattangmat non-cat math folder.
2/
you need to write the problem itself!

please re-post this thread (a) in the correct folder, and (b) with the actual problem you're asking about.
thanks.