We know Distance formula is Distance = speed * time
![time = (distance)/(speed)](https://img.qammunity.org/2019/formulas/physics/middle-school/n8eop8vh1ri72kcpphhbhbb9whdfxqqg91.png)
Let, the distance Omar traveled = d and speed is 10 miles per hour
Time taken t for the first trip
![t = (d)/(10)](https://img.qammunity.org/2019/formulas/mathematics/college/27e8frt6z66rw2n85whxh2eugrmi9c7vzi.png)
Let, the distance Omar traveled = d and speed is 30 miles per hour
Time taken t for the return trip
![t = (d)/(30)](https://img.qammunity.org/2019/formulas/mathematics/college/36cgdjkzetukomp000wc2fdxf1sdxnxu5d.png)
Total time taken =
![(d)/(10) + (d)/(30) = (3d+d)/(30) = (4d)/(30)](https://img.qammunity.org/2019/formulas/mathematics/college/coct03x7wj6vn4de5ua93pyeuj929mrb1e.png)
Now,
![Average \ speed = (total \ distance)/(total \ time \ taken)](https://img.qammunity.org/2019/formulas/mathematics/college/bqyxrwubcydigdtmwyc2a6uy0qy2h7jase.png)
Total distance = d+d= 2d
Average speed =
=
![2d * (30)/(4d) = 15](https://img.qammunity.org/2019/formulas/mathematics/college/nm67qjpbqmp8r20r1boo4xprpo0wc3hh2t.png)
His average speed for the entire trip from home to the gift store and back is
15 miles per hour