45 miles per hour is the answer.

distance travelled first 1 hr = (1/2 hr * 30) = 15 miles (in 1/2 hr)
time taken for 30 miles @ 60 miles/hr = (30/6) = 1/2 hr

Total time taken = 1/2 + 1/2 = 1 hour
Total distance travelled = 15 + 30 = 45 miles
Avg speed = 45 miles/hr

If my memory serves me correctly, I think the problem is supposed to say:"Susan drove an average speed of 30mph for the first 30 miles of a trip."

I understood this way. Susan drove x miles for first 30 minutes @ 30 miles/hr. ==> x = (distance x time) = (1/2 hr x 30 miles/hr) = 15 miles.

==> 15 miles in 1/2 hour.

Later, drove further 30 miles @ 60 miles per hour.
==> time = (distance / speed) = (30/60) = 1/2 hour.

So she travelled 15 miles in first 1/2 hour and further 30 miles in the next 1/2 hour.

So she travelled 45 miles in 1 hour. Hence avg speed = 45 miles / hour.

truplayer256 wrote:If my memory serves me correctly, I think the problem is supposed to say:"Susan drove an average speed of 30mph for the first 30 miles of a trip."

I keep getting 45mph as well using the same process but for some reason the answer in 40mph. Can anyone shine some light on that? Otherwise since this is a Kaplan problem it is possible that they made a mistake (as they are known to do).

truplayer256 is right; you read the problem incorrectly.

It's supposed to say that Susan drove at an average speed of 30 mph in the first 30 miles of the trip. That's why you're getting the wrong answer of 45 mph.