The definition of magnetic flux is the integral of the magnetic field over a surface area.
F = int_surf ( B dot dS )
In general, S can be any oddly-shaped surface. In your problem statement, all of the flux lines are normal to the surface. This means that you can visualize the answer to the problem by mapping (stretching) the surface into a rectangle shape with surface area 20 cm^2. Just imagine that all the field lines morph with the surface so that they remain normal, and the field density remains 15 W/m^2.
Now, your dealing with a geometry that is simple. B is constant, so take it out of the integral:
F = B int_surf ( 1 dot dS ), where i is the unit vector normal to the new surface. But, int_surf ( 1 dot dS ) is just the area of the surface, 20cm^2.
Thus, F = 20 cm^2 * 15 Web/m^2 * 1 m^2 / ( 100 cm )^2
F = 30 mWeb