Time speed distance formula - require more clarity

[himanshu.hpr]

This is a general query --- if for a total distance d a person walks the first 10 miles with a speed of x mph and the rest of the distance with a speed of 2x mph....


We calculate. ---> average speed = d/(10/x+(d-10)/2x) or = total distance / total time taken .

Now the question is wether we can calculate total time by ------>

Time taken to complete whole journey = d/(x+2x) or = total distance / summation of individual speeds..

Pl. comment on this formulas validity and wether it can be used as a standard result.

[RonPurewal]

We calculate. ---> average speed = d/(10/x+(d-10)/2x) or = total distance / total time taken .

the green part is total time. in fact, you've pointed that out yourself.

Now the question is wether we can calculate total time by ------>

Time taken to complete whole journey = d/(x+2x) or = total distance / summation of individual speeds..

no, definitely not. try to think about this intuitively for a second -- it doesn't make any sense to add the speeds here. nothing is ever traveling at a speed of (x + 2x), so you should know something is wrong if you ever use that expression for speed.

the good news, though, is that you've already calculated the total time!
just look at the green stuff above -- no need to do the same work again.

[himanshu.hpr]

[quote="RonPurewal"]
Thanks for the explanation.

[tim]

:)