Review. Oxygen at pressures much greater than 1 atm is toxic to lung cells. At sea-level the partial pressure of oxygen is 0.21 atm. Assume a deep-sea diver breathes a mixture of oxygen and helium (He). By weight,what ratio of helium to oxygen must be used if the diver is at an ocean depth of 50.0 m? Take the density of water to be roughly 1000 kg/m^3

The other answer I saw for this problem on here uses P = n/M, which is an equation I wasn't able to derive nor find anywhere on the internet or in my textbook. If someone would like to also explain why it's true or not use it to solve this problem, it would be greatly appreciated.