Some hypothetical metal is known to have an electrical resistivity of 4 x 10-8 (-m). Through a specimen of this metal that is 25 mm thick is passed a current of 30 A; when a magnetic field of 0.75 tesla is simultaneously imposed in a direction perpendicular to that of the current, a Hall voltage of -1.26 x 10-7 V is measured. Compute (a) the electron mobility for this metal, and (b) the number of free electrons per cubic meter.