QR problem

[dentalknight]

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ADA Sample Q's
A person travels to work at an average speed of 40 mph, and returns home at 60mph. Which of the following is the average speed for his entire trip?
A) 45
B) 46
C) 48
D) 52
E) 54

at a quick glance i thought 50, but thats clearly not the case. i reread the question, but i dont get what to do for it

 

[DDSguyLA]

speed = distance / Time

pick a number for distance and handle it that way..

 

[DDSguyLA]

heres a better look at it.

Know this... Average Speed = Total Distance / Total Time

Also know this... Distance = Rate (speed) x Time

{you can change between rate, time and distance easily}


So like I said pick a distance.. lets say 100 miles. Since the distance doesnt change from his home to work and back we use 100 miles for both ways.

Now, Change the second formula to find Time...

Time = Distance / Rate
----------------------------

Going to work

Time = 100 miles / 40 which gives you (2.5) for time


Coming back from work

Time = 100 miles / 60 which gives you (1.66) for time

----------------------------

Now plug those in the actual average speed formula


Av Speed = Total Distance (100+100) / Total Time (2.5 + 1.66) = 48


48 is your average speed...

Hope this helped.

Good luck

 

[Streetwolf]

Choose a distance like 120 miles because 40 and 60 go into it evenly. Then you have 3 hours one way and 2 hours back. With 240 total miles and 5 hours, your average speed is 48 mph (240 miles / 5 hours).