When a pumper is connected to a hydrant and the first line is opened, what does a drop in residual pressure of 20% indicate?

Answer: C — Two times the amount of water (one additional setup).

When a pumper is connected to a hydrant and the first line is opened, a drop in residual pressure of 20% indicates that the system can support the addition of more water flow. A 20% drop in residual pressure typically corresponds to a situation where the conditions have changed significantly due to the additional demand placed upon the water source. This specific drop indicates that the pump is still able to maintain adequate pressure while supplying the water necessary for firefighting operations. In this context, the drop in residual pressure reflects that the system is under increased demand but is still functioning effectively, meaning that the water supply can accommodate additional setups without a critical loss in pressure. In practical terms, this operational pressure tells firefighters that they can safely add another water line. The figure of "two times the amount of water" translates to the capacity of the hydrant to support not just the existing flow but also accommodate more water without compromising performance. Thus, understanding the relationship between residual pressure and available water flow is crucial for effective firefighting tactics.