4 little piggies mom said:
Suppose cities a and b are a distance d apart. You travel from a to b at an average speed of x miles per hour, and then you return to a at an average speed of y miles per hour. If z is the average speed of the whole trip, which is true?
a. z= (x+ y)/2
b. z= (d/x) + (d/y)
c. 1/z = 1/2 (1/x + 1/y)
d. 1/z = 1/x + 1/y
e. none of these~
she's got C but I think b
Maybe it's just me, but wouldn't it be more effective if your daughter posted her
own questions and showed her
own work? That way we'd have a much better idea of
her level of understanding.....
That said, here is some information that may help you solve this problem:
distance = rate * time
time = distance/rate
Average rate = (total distance)/(total time)
If the distance between cities A and B is "d" miles, then a round trip from A to B and back would cover 2d miles.....that's the total distance.
Going from A to B at x mph takes d/x hours.
Returning from B to A at y mph takes d/y hours.
Total time, then, is (d/x) + (d/y) hours.....
Now, can you take it from here?
