Bob drove to the store( a distance fo 10.5) miles at ana average speed of 35 mile per hours. On the way home, he got stuck in traffic, and he only averaged 15 miles per hour along the same route. What was his average speed for the round trip ?

time=distance/speed, so we have

10.5/35 + 10.5/15 = 21/s

21 mph

To find the average speed for the round trip, you need to calculate the total distance traveled divided by the total time taken.

Let's first calculate the time it took for Bob to drive to the store.
Distance = 10.5 miles
Speed = 35 miles per hour

Time = Distance / Speed
= 10.5 miles / 35 miles per hour
= 0.3 hours

Now, let's calculate the time it took for Bob to drive back home.
Distance = 10.5 miles
Speed = 15 miles per hour

Time = Distance / Speed
= 10.5 miles / 15 miles per hour
= 0.7 hours

To find the total time taken for the round trip, we add the time taken to go to the store and the time taken to return back home.
Total time = Time to go + Time to return
= 0.3 hours + 0.7 hours
= 1 hour

Now, let's calculate the total distance traveled for the round trip.
Distance to the store = 10.5 miles
Distance back home = 10.5 miles

Total distance = Distance to the store + Distance back home
= 10.5 miles + 10.5 miles
= 21 miles

Finally, let's calculate the average speed for the round trip.
Average speed = Total distance / Total time
= 21 miles / 1 hour
= 21 miles per hour

Therefore, Bob's average speed for the round trip is 21 miles per hour.