12. A Carnot engine operates between 27°C and -23°C. What is the efficiency? (A) 100 % (B) 83 % (C) 54 % (D) 47 % (E) 17 %

5. A boy stands on a board placed on a ice (frictionless). The mass of the boy including the board is 20 kg. When he throws a ball with mass of 1 kg to the right at the speed of 10 m/s (Fig. 2), what is the velocity of the boy and board? (A) 10 m/s to the left (B) 10 m/s to the right (C) 0.5 m/s to the left (D) 0.5 m/s to the right (E) 0 m/s