S.O.S. Mathematics CyberBoard

I was just stuck on a part of the derivation of this equation.

Dt = Dd + Dv Total Density is Desity of Dry air plus Density of Moist Air

Dt = [Pd/Rd*T] + [Pv/Rv*T] This is understood, but which equations and how are they manipulated to come to the final equation of:

Dt = [P/Rd*T]*[1- [.378*Pv/P]]


P = Total Air Pressure
Pv= Vapor Pressure
Pd = Dry Air pressure
Rv=gas constant for both water vapor
Rd=gas constant for dry air
T=Absolute Temperature.

If someone could guide me through the steps to arrive at the final equation, please don't hesitate. Thanks

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