Answer:
83.2
Explanation:
Tank constant = 1.56
Safe Residual Pressure = 200
Flow = 15
Pressure = 1000 PSI
Duration = ?
1.56 * (1000-200) = 1248
1248/ 15 = 83.2
Formula: (Tank constant * (Tank pressure - Safe Residual Pressure)) / Flow Rate
Here's a calculator to test it for yourself, http://www-users.med.cornell.edu/~spon/picu/calc/o2tankd.htm