Plz see the pic below and explain the following doubts:

What principle says the first eqn:

Q$1.$ How can mass flow rate of $($dry air$)$ m$=$ volumetric rate$/$ specific volume of air $($ at $1)$

Q$2.$ How is nR $=$ R$/$ Ma$?($ how can they be interchangeable $)($ this is with context to ideal gas equation$)$