Final answer:
The absolute pressure at 20 meters underwater is 3 ATM, which includes the atmospheric pressure at sea level plus the pressure due to the water above.
Step-by-step explanation:
The absolute pressure at a depth of 20 meters underwater can be calculated using the concept of pressure in fluids. Atmospheric pressure at sea level is considered to be 1 ATM (or 1 atmosphere), and for every 10 meters of water depth, the pressure increases by approximately 1 ATM due to the weight of the water above. Therefore, at 20 meters, the pressure due to the water alone would be 2 ATM, and when adding the atmospheric pressure at sea level, the total absolute pressure would be 3 ATM.