In a scale of 1 to 40 000 how many centimeters on a map are represented by 2km on the ground?

You will see a distance of 5 cm on your map represent a 2 km distance across the landscape. Your 2 km is 2,000 meters, and there are 100 cm in a meter. That makes for 200,000 cm in 2 km. If you "reduce" or scale it down by a factor of 40,000, you'll get 200,000 / 40,000 = 5 cm.