A motorist drives 150km from one city to another in 2.5h, but makes the

return trip in only 2.0h. What are the average speeds for

a. Each half of the round-trip and

b. The total trip?

To find average speed, you do total distance / total time . For this question, I found the individual speed as 17m/s and 21m/s. and so if I do v1 + v2 ) / 2 , i happened to get the answer for b. but the actual formula to use is total distance / time right.. i'm conufsed about that.

Second, for average acceleration.

t = 1 d= 10

t= 2 d= 30

to find the avg a for this, why can't you do (30-10) / (2-1) / (2-1 ) but d2/t2 - d1/t2 / t2 - t1 , or why is it not total velocity / total time just like what average speed or velocity is.

Third, why is avg vel = total displacement / total time but not change in displacement / change in time?

Please answer this !! thank you very much!