Velocity is a vector and hence when the ball strikes a wall and rebounds, it changes direction and this change in direction should also be considered when thinking about velocity change.
Let the direction of towards wall is positive, then the direction of rebound is negative. The velocity of ball going towards the wall is 20 m/s, the velocity of rebound will then be -10 m/s.
Change in velocity = initial -final velocity = 20 - (-10) m/s = 30 m/s
Hope this helps