Solution to task 458 from Chertov´s for extramural stud

458. In a uniform magnetic field perpendicular to the lines of induction there is a flat contour with an area S = 100 cm2. Maintaining a constant current I = 50 A in the loop, it was moved from the field to the area of space where the field is absent. Determine the induction B of the magnetic field if the work A = 0.4 J was performed while moving the contour.