Answer: 20 yards
=========================================================
Explanation:
They start on the 20 yard line. After the first play, they gain 10 yards and move to the 20+10 = 30 yard line.
After the second play, they must have gained 20 yards because 30+20 = 50.
Or you can think of it like 50-30 = 20, which shows the distance from the 30 yard line to the 50 yard line is 20 yards.
All of this assumes that the Cowboys started on their side of the field.
