Cross-reference with the PADI Guide To Teaching for how to do this skill and always use the dive slate listing the skills conduct which is on the reverse of the Dive Flex Skills from the PADI Open Water Course.

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Fail-safe regulators are designed to free flow in a fully open position upon failure rather than close off the air supply.

This means that even if the regulator fails it will supply air and is the very reason we practice free-flow regulator skills in the PADI Open Water Diver Course.

Although Haemoglobin transports oxygen around the body it bonds with carbon monoxide much more readily.

This means that smoking prior to a dive may be even more hazardous to a divers health than a non-diver. 

Flying after diving considerations are always listed on the reverse of the RDP under Table 3 and on the inside lid of the eRDml.

Although a common misconception that you must always have a 24-hour waiting period before flying after any dive it isn't always the case.

I would always recommend following the no-fly time assigned by your personal dive computer as this is more reliable as it knows the dive profile you did. 

Always take the object to the surface in these types of questions. This will simplify things for you no end as you'll have a base number to revert back to if things don't seem to be right in the second part of the calculation.

If the object is 25% full at 4ATA - 30m/sw - it will be 4 times larger at the surface - 100%

Now we just have to take the flexible container down to 15m/sw with would be 2.5ATA - 100/2.5=40