Question 1011507

May you please help me with this question?

The distance from Calgary to red deer is approximately 140 km. Some friends leave Calgary at noon, drive to red deer, have a half-hour break, and then return to Calgary. They arrive at 3:15 p.m. Traffic is busier on the return trip, so their average speed is 10km/h slower than on the outbound trip. What is the average speed of the round trip? 

*hint* the textbook answer is 107 km/h 

Steps/Calculations please? 
<pre>Difference in speeds is irrelevant
Total distance: 2(140), or 280 km
Total time: {{{2&3/4}}} ({{{3&1/4 - 1/2}}}) hours 
Average speed: {{{Total_distance/Total_time}}} -------> {{{280/(2&3/4)}}}
This doesn't result in an AS of 107 km/h. You need to check why!