We know Distance formula is Distance = speed * time

Let, the distance Omar traveled = d and speed is 10 miles per hour
Time taken t for the first trip

Let, the distance Omar traveled = d and speed is 30 miles per hour
Time taken t for the return trip

Total time taken =

Now,

Total distance = d+d= 2d
Average speed =
=

His average speed for the entire trip from home to the gift store and back is
15 miles per hour