What is the surface interval time required for a dive to 30 meters (98 feet) for 30 minutes if the first dive is to 20 meters (66 feet) for 40 minutes?

The surface interval time required is determined by various factors, including the depth and duration of the dives, as well as the need for residual nitrogen to off-gas from the body before making subsequent dives.

After the first dive to 20 meters for 40 minutes, divers typically accumulate nitrogen in their tissues, which requires a specific surface interval before the next dive to allow for adequate off-gassing. The second dive to 30 meters for 30 minutes poses a greater risk of nitrogen accumulation due to the increased depth and the fact that this dive follows shortly after a deeper, longer dive.

Using standard dive tables or dive computer calculations helps to establish the required surface interval based on these factors. In this case, a 60-minute surface interval is often prescribed to ensure sufficient nitrogen elimination, particularly since the second dive involves both a deeper depth and a considerable dive duration. This time allows the body to adequately reduce nitrogen levels and minimize the risk of decompression sickness in subsequent dives.

Thus, a surface interval of 60 minutes is appropriate, making it the correct answer to the question.