The emergency breathing gas that must be on site: divers times minutes times breathing rate times absolute pressure.
The formula: Divers x minutes x rate x absolute pressure. Change any box and the full working updates. For study only, never for dive planning.
The IMCA ADS, BDS and LST papers ask this as a four-option question, with a calculator on screen. A free account gives you 15 practice questions for each exam, each with the worked answer and the document it comes from.