math

[BrownieDDD]

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a person running a mile reached the halfway point in 1 min and 45 sec. If the person's avg speed for the entire mile was 15 miles per hour, how much time did it take to run the second half mile?

a. 1 min 30 sec
b. 1 min 45 sec
c. 2 min 15 sec
d. 3 min 15 sec
e. 3 min 30 sec



here's a caveat: don't ever take the averages of the rate; 0.5 (rate of first leg + rate of second leg)
There is a better formula, more correct to use..( see my way of solving for it)
answer : C

i got the answer doing this: Avg rate = total distance/total time
15 (1/3600)= 1/ 105 + X
solved for X
X= 135 sec= 2 min 15 sec

But Kaplan had a really interesting and FASTER way to solve; 15 mi / hr= 15 mi/60 min= 1 mi/4 min
Since the person is running a mile and stopped halfway at 1 min 45 sec,
Second half time= 4min - 1 min 45 sec= 2 min 15 sec

So sorry this is so long but here is what I am getting at...In kaplan's solution...if the person stopped say 2/3 of the way with time 1 min 45 sec, can the answer be 4-1 min 45 sec? Or does their way only work if you stop half way

 

[ijk90825]

2/3 still works the same.
It takes 4 minutes total anyway you run it, even if you run first 0.9 miles in 1 min 45 sec it'll take him 2 min 15 sec to finish the last 0.1 mile to make that 15 mile/hr