From his house, David walks 70 steps forward to the East then without reversing his back, he walks 30 steps backward. Then he reverses his back and walks 20 steps forward. What is the distance (in feet) between his final position and his house if one step measures 2 feet?

David faces east and walks 70 steps in the east direction. 
After this, without turning his back, David walks 30 steps in reverse direction. Landing him 40 steps east of the starting point, facing eastwards. 
After this he reverses his back, and is now facing west towards his starting point. He then walk 20 steps in the direction he is facing i.e, 20 steps towards the starting point. Making his final position 20 steps east of the starting point. 
1 step measures 2 feet, so 20 steps would measure 40 meters. 
Therefore, David is 40 meters east of the starting point. 

Hence, Option C is the correct answer.