At Canada’s Wonderland, Hideiki and Nicolas decide to go on the bumper cars. The bumper cars have a mass of 320 kg each while Nicolas has a mass of 72 kg and Hideiki has a mass of 63 kg. However, Hideki, always obsessed with Physics, has brought with him a stack of his favourite Physics books. Although riveting reading, they are exceptionally large and heavy. On the ride, Nicolas uses his amazing NASCAR driving skills and smashes into a stationary Hideiki in a head-on elastic collision. After the collision, Hideiki’s car moves with a speed that is 97% of Nicolas’s original speed. Assuming all surfaces are frictionless, what is the mass of Hideiki’s books?