James and Sarah stand on a stationary cart with frictionless wheels. The total mass of the cart and riders is 130 kg. At the same instant, James throws a 1.0 kg ball to Sarah at 4.5 m/s, while Sarah throws a 0.50 kg ball to James at 1.0 m/s. James's throw is to the right and Sarah's is to the left. 

a. While the two balls are in the air, what are the speed and direction of the cart and its riders? 

b. After the balls are caught, what are the speed and direction of the cart and riders?