Answer
The magnitude of the electric field at B is $20~N/C$
Work Step by Step
The density of field lines in any region is proportional to the magnitude of the electric field in that region.
Note that the electric field lines at B have twice the separation of those at A.
Since the magnitude of the electric field at A is $40~N/C$, the magnitude of the electric field at B is $20~N/C$
