Starting at home, Luis traveled uphill to the gift store for 50minutes at just 6 mph. He then traveled back home along the same path downhill at a speed of 12mph.

Average speed for the entire trip, both ways, is

                (Total distance) divided by (total time) .

We don't know the distance from his house to the gift store,

and we don't know how long it took him to get back.

We'll need to calculate these.

-- On the trip TO the store, it took him 50 minutes, at 6 mph.

-- 50 minutes is 5/6 of an hour.

-- Traveling at 6 mph for 5/6 of an hour, he covered 5 miles.

-- The gift store is 5 miles from his house.

-- The total trip both ways was 10 miles.

-- On the way BACK home from the store, he moved at 12 mph.

-- Going 5 miles at 12 mph, it takes  (5/12 hour) = 25 minutes.

Now we have everything we need.

Distance:

        Going:       5 miles

        Returning:  5 miles

        Total         10 miles

Time:

        Going:        50 minutes

        Returning:   25 minutes

        Total:          75 minutes  =  1.25 hours  

        Average speed for the whole trip =

                      (total distance) / (total time)

                  =      (10 miles)  /  (1.25 hours)

                  =       (10 / 1.25)  miles/hours

                  =          8 miles per hour

Step-by-step explanation:

8121 because

130,000 divided by 16 is 8121