Cave Creatures

Deep in a dark cave, live Archie Arachnid and Barbara Bat. When Barbara hangs down from their favorite stalactite and Archie is on the cave ceiling, the distance between their heads is 42cm. When they switch places the distance is only 20cm. How long is the stalactite? The answer, including units, can be used as a promo code for an escape room experience. The code is valid until November 1, 2024.