AAPL wrote:Jerry travels 8 miles at an average speed of 40 miles per hour, stops for 10 minutes, and then travels another 20 miles at an average speed of 60 miles per hour. What is Jerry's average speed, in miles per hour, for this trip?

A. 40
B. 42.5
C. 44
D. 50
E. 52.5

Your solution is perfect!

That said, another approach is to focus on the 2nd part of the trip.

In the first part of the trip, Jerry travels 8 miles at an average speed of 40 miles per hour

In the 2nd part of the trip, Jerry stops for 10 minutes, and then travels another 20 miles at an average speed of 60 miles per hour
Let's work out the average speed during the 2nd part of the trip.
Average speed = total distance/total time
We know that, during the 2nd part of the trip, Jerry traveled 20 miles, but we don't know the travel time.

Time = distance/speed
= (20 miles)/(60 mph) = 1/3 hours = 20 minutes

So, the TOTAL time spend during the 2nd part of the trip = (time spent stopping) + (time spent at 60 mph)
= 10 minutes + 20 minutes
= 30 minutes
= 0.5 hours

So, Average speed (during the 2nd part of the trip) = total distance/total time = 20 miles/0.5 hours = 40 mph

So, in the 1st part of the trip, Jerry traveled at a speed of 40 mph, AND in the 2nd part of the trip, Jerry traveled at a speed of 40 mph.
Since Jerry's average speed was 40 mph for each part of the trip, the average speed for the ENTIRE TRIP must be 40 mph

Answer: A

Cheers,
Brent

AAPL wrote:Jerry travels 8 miles at an average speed of 40 miles per hour, stops for 10 minutes, and then travels another 20 miles at an average speed of 60 miles per hour. What is Jerry's average speed, in miles per hour, for this trip?

A. 40
B. 42.5
C. 44
D. 50
E. 52.5

Using the formula Average = total distance/total time, we have:

Average = 28/(8/40 + 1/6 + 20/60)

Average = 28/(1/5 + 1/6 + 1/3)

Average = 28/(6/30 + 5/30 + 10/30)

Average = 28/(21/30) = (30 x 28)/21 = (30 x 28)/(3 x 7) = 10 x 4 = 40.

Answer: A