NASA uses a launch clock to measure time before and after lift-off. It takes 2 hours and 15 minutes of preparations on base, 20 minutes to travel to the launch pad, and 40 minutes of pre-launch checks once in the spacecraft. When should the astronauts begin preparation?

Answer: 3 hours 15 minutes, because all the preparation times add to 3 hours and 15 minutesStep-by-step explanation:Since, The astronaut will begin preparation from the time when the preparation on the base starts,Thus, his total time = Time in preparation on  base + Time in travelling to the launch pad + Time in pre-launching check once in the space craft.= 2 hours 15 minutes + 20 minutes + 40 minutes= 3 hours 15 minutes.Thus, he should begin preparation before 3 hours 15 minutes.