What speed of the ship, 64m long does this correspond to?

Question

A 1.0 m long model of a ship is towed at a speed pf 81 cm/s in a towing tank. To what speed of the ship, 64m long does this correspond to?                  

a. 7.20 m/s                   b. 6.48 m/s

c. 5.76 m/s                   d. 3.60 m/s