A straight conductor is moved at a velocity of 10m/s in magnetic field of flux density 0.8T. If the induced E.M.F is 20V.calculate the length of the conductor?

Use this equation, which I'm sure is in your text book:

EMF(Volts) = Velocity(m/s) x B-field (T) x Length (m)

This assumes that the wire is moving perpendicular to the magnetic field, which isn't explicitly stated but you can probably assume.