Average Speed

Each statement by itself is sufficient.

Take 1: Mary drove for 30/60 = half an hour with an average speed of 60 miles/h and then she drove for 20/50 = 0.4 hours with an average speed of 50 miles/h. This means she drove 50 miles in 0.9 hours, which gives us an average speed of 50/0.9 = 55.56 miles/h

Take 2: Total distance = 50 and total time = 54 mins = 0.9 h. So average speed = total dist/total time = 50/0.9 = 55.56 miles/h.

Answer D.

So, for the first statement - if this was a PS question - is there a shortcut?

Something like the weighted averages, or speed of a boat upriver & downriver... where the distance of each part of the trip is the same. Is there a similar shortcut here that I use instead of finding Total time and Total distance.

Thanks alot.

Well, trust me, it's pretty short if you just write down the equations. I don't really know of any shorter method, but maybe someone else can help.

kayser wrote:So, for the first statement - if this was a PS question - is there a shortcut?

Something like the weighted averages, or speed of a boat upriver & downriver... where the distance of each part of the trip is the same. Is there a similar shortcut here that I use instead of finding Total time and Total distance.

Thanks alot.

A great way to attack multiple-part speed questions is with a chart.

Across the top of the chart, write the formula, D = r * t.
Down the left side of the chart, write the different parts of the journey, e.g.:

part 1
part 2
total

or

uphill
downhill
total

So, for this question, your chart will look something like this (excuse my poor formatting!):

.................Distance....=...........Rate.........*...........Time

Part 1...........30........................60...............................

Part 2...........20........................50...............................

Total.............50........................x................................

The chart allows you to quickly assimilate the information you're given and plan your route to solve the question.

In this case, we'd see that we can calculate the times for the individual legs, then if we add those together we get total time. Since:

Avg Rate = Total Distance/Total Time,

we'd then have everything we need to calculate the final answer.

The chart is simply a visualization of the equations to which Dana referred, but a lot of people love it because it lays out all the pieces so neatly.